Partial Thromboplastin Time, Activated (aPTT, Partial Thromboplastin Time [PTT])
Critical ValuesIndications
The PTT test is used to assess the intrinsic system and the common pathway of clot formation. It is also used to monitor heparin therapy.
Test Explanation
Hemostasis and the coagulation system represent a homeostatic balance between factors encouraging clotting and factors encouraging clot dissolution. The first reaction of the body to active bleeding is blood vessel constriction. In small vessel injury this may be enough to stop bleeding. In large vessel injury, hemostasis is required to form a clot that will durably plug the hole until healing can occur. The primary phase of the hemostatic mechanism involves platelet aggregation to blood vessel (see Figure 2-12 on p. 167). Next, secondary hemostasis occurs. The first phase of reactions is called the intrinsic system. Factor XII and other proteins form a complex on the subendothelial collagen in the injured blood vessel. Through a series of reactions, activated factor XI (XIa) is formed and activates factor IX (IXa). In a complex formed by factors VIII, IX, and X, activated X (Xa) is formed.
At the same time the extrinsic system is activated and a complex is formed between tissue thromboplastin (factor III) and factor VII. Activated factor VII (VIIa) results. VIIa can directly activate factor X. Alternatively, VIIa can activate IX and X together.
The final step is a common pathway in which prothrombin is converted to thrombin on the surface of the aggregated platelets. The main purpose of thrombin is to convert fibrinogen to fibrin, which is then polymerized into a stable gel. Factor XIII crosslinks the fibrin polymers to form a stable clot.
Almost immediately three major activators of the fibrinolytic system act on plasminogen, which had previously been absorbed into the clot, to form plasmin. Plasmin degenerates the fibrin polymer into fragments that are cleared by macrophages.
The PTT evaluates factors I (fibrinogen), II (prothrombin), V, VIII, IX, X, XI, and XII. When the PTT is combined with the prothrombin time, nearly all of the hemostatic abnormalities can be recognized. When any of these factors exists in inadequate quantities, as in hemophilia A and B or consumptive coagulopathy, the PTT is prolonged. Because factors II, IX, and X are vitamin K–dependent factors, biliary obstruction, which precludes GI absorption of fat and fat-soluble vitamins (e.g., vitamin K), can reduce their concentration and thus prolong the PTT. Because coagulation factors are made in the liver, hepatocellular diseases will also prolong the PTT.
Heparin has been found to inactivate prothrombin (factor II) and to prevent the formation of thromboplastin. These actions prolong the intrinsic clotting pathway for approximately 4 to 6 hours after each dose of heparin. Therefore heparin is capable of providing therapeutic anticoagulation. The appropriate dose of heparin can be monitored by the PTT. PTT test results are given in seconds along with a control value. The control value may vary slightly from day to day because of the reagents used.
Recently activators have been added to the PTT test reagents to shorten normal clotting time and provide a narrow normal range. This shortened time is called the activated PTT. The normal aPTT is 30 to 40 seconds. Desired ranges for therapeutic anticoagulation are 1.5 to 2.5 times normal (e.g., 70 seconds). The aPTT specimen should be drawn 30 to 60 minutes before the patient's next heparin dose is given. If the aPTT is less than 50 seconds, therapeutic anticoagulation may not have been achieved and more heparin is needed. An aPTT greater than 100 seconds indicates that too much heparin is being given; the risk for serious spontaneous bleeding exists when the aPTT is this high. The effects of heparin can be reversed by the parenteral administration of 1 mg of protamine sulfate for every 100 units of the heparin dose.
Heparin's effect, unlike that of warfarin, is immediate and short lived. When a thromboembolic episode (e.g., pulmonary embolism, arterial embolism, thrombophlebitis) occurs, immediate and complete anticoagulation is most rapidly and safely achieved by heparin administration. This drug is often given during cardiac and vascular surgery to prevent intravascular clotting during clamping of the vessels. Often small doses of heparin (5000 units subcutaneously every 12 hours) are given to prevent thromboembolism in high-risk patients. This dose alters the PTT very little, and the risk for spontaneous bleeding is minimal.
Interfering Factors
• Drugs that may prolong PTT test values include antihistamines, ascorbic acid, chlorpromazine, heparin, and salicylates.
Clinical Priorities
• The PTT is used to monitor heparin therapy. Heparin's effect is immediate and short lived.
• If too much heparin is given, its effects can be reversed by parenteral administration of protamine sulfate.
• Patients receiving heparin need to be evaluated for bleeding tendencies. These include bruising, petechiae, low-back pain, and bleeding gums. Blood may be detected in the urine and stool.
Procedure and Patient Care
After
• Apply pressure or a pressure dressing to the venipuncture site.
• Assess the venipuncture site for bleeding. Remember, if the patient is receiving anticoagulants or has coagulopathies, the bleeding time will be increased.
• Assess the patient to detect possible bleeding. Check for blood in the urine and all other excretions and assess the patient for bruises, petechiae, and bleeding gums.
• If severe bleeding occurs, note that the anticoagulant effect of heparin can be reversed by parenteral administration of protamine sulfate.
Test Results and Clinical Significance
Increased Levels
Congenital clotting factor deficiencies (e.g., von Willebrand disease, hemophilia, hypofibrinogenemia): These hereditary illnesses are associated with very little, if any, of the respective clotting factors. As a result, the PTT is prolonged.
The liver makes most of the clotting factors. For synthesis of some of those clotting factors, vitamin K is required. In the above illnesses the clotting factors of the intrinsic system and common pathways are inadequate in quantity. As a result, the PTT is prolonged.
Disseminated intravascular coagulation (DIC): Key clotting factors involved in the intrinsic system are consumed.
Heparin administration: Heparin inhibits the intrinsic system at several points. As a result, the PTT is prolonged.
Coumarin administration: Although coumarin has a greater impact on the prothrombin time, it does inhibit the function of factors II, IX, and X. As a result, the PTT is prolonged.
Parvovirus B19 Antibody
Indications
This test is performed on children who have vague symptoms of fever, arthralgias, and rash suggestive of erythema infectiosum. It is also becoming a part of routine testing for proposed organ donors.
Test Explanation
The parvovirus group includes several species-specific viruses of animals. Parvovirus B19 is known to be a human pathogen. Many of the severe manifestations of B19 viremia relate to the ability of the virus to infect and lyse red blood cells (RBCs) precursors in the bone marrow. The name “B19” was derived from the code number of the human serum in which the virus was discovered.
Erythema infectiosum is the most common manifestation of B19 infection and occurs predominantly in children. This pathogen is also referred to as “fifth disease,” because it was classified in the late nineteenth century as the fifth in a series of six exanthems of childhood. This infection is also sometimes referred to as “academy rash.” The typical presentation is a self-limiting, mild illness with a low-grade fever, malar rash, and occasionally arthralgia. Normally the rash begins on the face and may also develop on the arms and legs. Outbreaks of erythema infectiosum appear most often during the winter and spring months.
Parvovirus B19 has also been associated with a number of other clinical problems, including:
• Flulike illness associated with joint inflammation, rash, and occasionally purpura in young adults.
• Increased risk for abortion or stillbirth caused by hydrops fetalis and fetal loss in some infected pregnant women.
• Transient aplastic crisis in patients with chronic hemolytic anemia. In immunocompromised patients (acquired immunodeficiency syndrome [AIDS] patients, organ donors), this virus can be so severe as to cause aplastic anemia and bone marrow failure. With increasing frequency, this antibody test is being used for all potential organ donors.
• Chronic severe anemia in patients with immunodeficiency related to infection with human immune deficiency virus (HIV), congenital immunodeficiency, acute lymphocytic leukemia during maintenance chemotherapy, and recipients of bone marrow transplants.
Because of the recently discovered spectrum of diseases caused by parvovirus B19, laboratory diagnosis has come into great demand. Serologic testing for parvovirus B19–specific IgM and IgG antibodies can be detected by enzyme-linked immunosorbent assay (ELISA) and indirect fluorescent antibody immunofluorescence methods. Acute infections can be determined by B19-compatible symptoms and the presence of IgM antibodies that remain detectable up to a few months. Past infection or immunity is documented by IgG antibodies that persist indefinitely with IgM antibodies. Fetal infection may be recognized by hydrops fetalis and the presence of B19 DNA in amniotic fluid or fetal blood.
Pepsinogen
Indications
This test is used to identify pernicious anemia (PA), gastric atrophy, or peptic disease. It is also used to identify precancerous changes in those at great risk for gastric carcinoma.
Test Explanation
Pepsinogens are secreted in the stomach and are made in the oxyntic gland mucosa of the proximal stomach. When exposed to gastric acid, pepsinogen is converted to pepsin, an active enzyme that is proteolytic and promotes digestion. Patients with gastric atrophy, pernicious anemia (PA), or those who have had gastrectomy have low levels of pepsinogen I. Pepsinogen I levels are slightly elevated in gastric ulcer, higher in gastroduodenal ulcer, and significantly elevated in duodenal ulcer. Patients with Zollinger-Ellison syndrome exhibit greatly elevated levels. Pepsinogen I has been used as a subclinical marker of increased risk for stomach cancer. Pepsinogen I can also be measured in the urine.
Pepsinogen II is made by oxyntic gland mucosa cells that are in the distal stomach and proximal duodenum. Because PA generally affects the proximal stomach, diminished levels of pepsinogen I with normal levels of pepsinogen II are strongly supportive of PA.
Pepsinogens are measured by immunoassay techniques in the blood and urine.
Procedure and Patient Care
Before
Explain the test to the patient.
Instruct the patient to fast for 10 to 12 hours before collection of the specimen.
Tell the patient that antacids or other medications affecting stomach acidity or gastrointestinal motility should be discontinued, if possible, for at least 48 hours before collection. Verify with the laboratory or health care provider.
Test Results and Clinical Significances
Pheochromocytoma Suppression and Provocative Testing (Clonidine suppression test [CST], Glucagon stimulation test)
Indications
These tests are used to identify pheochromocytoma when catecholamine levels are not assuredly diagnostic.
Test Explanation
In patients with significantly high blood pressure that is refractory to treatment, the diagnosis of pheochromocytoma (PH) is often considered. PHs usually arise from the adrenal glands and are often difficult to detect. Pheochromocytomas release chemicals called catecholamines, causing high blood pressure that is excessive and resistant to treatment. The definitive diagnosis of pheochromocytoma rests primarily on the demonstration of excessive catecholamine production, best achieved with a resting plasma catecholamine assay. When basal catecholamine plasma levels are excessive (norepinephrine >2000 pg/mL) in nonstressed patients, the diagnosis of PH is certain. However, when basal levels are far less than 2000 pg/mL, the diagnosis of PH is far less certain. Plasma catecholamine levels are not often helpful unless the blood specimen is obtained during a hypertensive paroxysm. Urine metanephrines (p. 975) are best tested at times other than hypertensive episodes.
When the diagnosis of PH is not certain, suppression and provocative tests may be necessary. Plasma catecholamines (epinephrine and norepinephrine) are particularly useful during suppression or provocative tests.
Normally, glucagon (less commonly metoclopramide and naloxone) is used as a provocative agent. Glucagon stimulates the release of catecholamines. In the presence of pheochromocytoma, the agents can cause the tumor to release excessive catecholamines into the bloodstream. The glucagon stimulation test has been superseded in recent years by the clonidine suppression test because it can provoke dangerous increases in blood pressure in patients with pheochromocytomas.
Clonidine is normally a potent suppressor of catecholamine production. Yet it has little to no suppressive effect on catecholamines in patients with pheochromocytoma. Suppressive testing is much safer than provocative testing because there is no real chance of a hypertensive paroxysm. The clonidine suppression test (CST) is nearly 100% accurate.
Potential Complications
Procedure and Patient Care
Test Results and Clinical Significance
Pheochromocytoma: This is a rare catecholamine-secreting tumor of the adrenal gland derived from chromaffin cells. These tumors can also arise outside the adrenal gland and are termed extraadrenal pheochromocytomas or paragangliomas. They may precipitate life-threatening hypertension or cardiac arrhythmias.
Related Test
Vanillylmandelic Acid (p. 975). This 24-hour urine test for VMA and catecholamines is performed primarily to diagnose hypertension secondary to pheochromocytoma. It is also used to detect the presence of neuroblastomas and other rare adrenal tumors.
Phosphate (PO4), Phosphorus (P)
Indications
This test is performed to assist in the interpretation of studies investigating parathyroid and calcium abnormalities. It is usually done to measure phosphate levels and ensure that adequate blood levels exist.
Test Explanation
Phosphorus in the body is in the form of a phosphate. Phosphorus and phosphate will be used interchangeably throughout this and other discussions. Most of the phosphate in the body is a part of organic compounds. Only a small part of total body phosphate is inorganic phosphate (i.e., not part of another organic compound). It is the inorganic phosphate that is measured when a “phosphate,” “phosphorus,” “inorganic phosphorus,” or “inorganic phosphate” is requested. Most of the body's inorganic phosphorus is intracellular and combined with calcium within the skeleton; however, approximately 15% of the phosphorus exists in the blood as a phosphate salt. The organic phosphate (not measured by this test) is used to synthesize part of the phospholipid compounds in the cell membrane, adenosine triphosphatase (ATP) for energy source in metabolism, nucleic acids, or enzymes (e.g., 2,3-diphosphoglycerate). The inorganic phosphate (measured in this test) contributes to electrical and acid-base homeostasis.
Dietary phosphorus is absorbed in the small bowel. The absorption is very efficient, and only rarely is hypophosphatemia caused by gastrointestinal (GI) malabsorption. Antacids, however, can bind phosphorus and decrease intestinal absorption. Renal excretion of phosphorus should equal dietary intake to maintain a normal serum phosphate level. Phosphate levels vary significantly during the day, with lowest values occurring around 10 AM and highest values occurring 12 hours later.
Phosphorus levels are determined by calcium metabolism, parathormone (parathyroid hormone [PTH]), renal excretion, and, to a lesser degree, intestinal absorption. Because an inverse relationship exists between calcium and phosphorus, a decrease of one mineral results in an increase in the other. Therefore, serum phosphorus levels depend on calcium metabolism and vice versa. The regulation of phosphate by PTH is such that PTH tends to decrease phosphate reabsorption in the kidney. PTH and vitamin D, however, tend to stimulate phosphate absorption weakly within the gut.
Hypophosphatemia may have four general causes: shift of phosphate from extracellular to intracellular, renal phosphate wasting, loss from the gastrointestinal tract, and loss from intracellular stores. Hyperphosphatemia is usually secondary to increased intake or an inability of the kidneys to excrete phosphate.
Interfering Factors
• Recent carbohydrate ingestion, including intravenous (IV) glucose administration, causes decreased phosphorus levels, because phosphorus enters the cell with glucose.
Laxatives or enemas containing sodium phosphate can increase phosphorus levels.
Drugs that may cause increased levels include methicillin, steroids, some diuretics (furosemide and thiazides), and vitamin D (excessive).
Drugs that may cause decreased levels include antacids, albuterol, anesthesia agents, estrogens, insulin, oral contraceptives, and mannitol.
Procedure and Patient Care
During
Test Results and Clinical Significance
Increased Levels (Hyperphosphatemia)
Hypoparathyroidism: Renal reabsorption is enhanced.
Renal failure: Renal excretion of phosphates is diminished.
Increased dietary or IV intake of phosphorus: The increased intake obviously leads to transiently elevated phosphate levels.
Acromegaly: Renal reabsorption is enhanced.
Bone metastasis: The phosphate stores in the bones are mobilized by the destructive bone tumors.
Sarcoidosis: Intestinal absorption of phosphates is increased because of the vitamin D effect produced by granulomatous infections.
Hypocalcemia: Calcium and phosphates exist in an inverse relationship. When one is elevated, the other is low.
Acidosis: When the pH is reduced, phosphates are driven out of the cell and into the bloodstream as part of a buffering system.
Decreased Levels (Hypophosphatemia)
Inadequate dietary ingestion of phosphorus: This is very rare, because phosphate reabsorption in the intestine is so efficient.
Chronic antacid ingestion: Antacids bind the phosphate in the intestine and preclude absorption.
Hyperparathyroidism: PTH increases urinary excretion of phosphates.
Hypercalcemia: Calcium and phosphate levels have an inverse relationship. When one is elevated, the other is low.
Chronic alcoholism: The pathophysiology of this observation is probably due to multiple causes. It may be in part nutritional and in part because of magnesium deficiency.
Vitamin D deficiency (rickets): Renal tubules fail to reabsorb phosphates.
Malnutrition: Rarely is malnutrition a cause of phosphate deficiency, because phosphate is so efficiently absorbed through the intestine. However, when malnutrition is associated with a deficiency of fat-soluble vitamins such as vitamin D, phosphate renal reabsorption is diminished. Phosphate levels decrease.
Alkalosis: Phosphate acts as a buffer. When pH increases, phosphate levels in the blood diminish because of an intracellular shift.
Phosphatidylinositol Antigen (PI-Linked Antigen)
Indications
The PI-linked antigen is useful for screening and confirming the diagnosis of paroxysmal nocturnal hemoglobinuria (PNH). It is also used to monitor the disease.
Test Explanation
Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired hematologic disorder of the bone marrow stem cell that is characterized by nocturnal hemoglobinuria, chronic hemolytic anemia, thrombosis, and pancytopenia, and in some patients by acute or chronic myeloid malignancies. These patients have dark urine caused by ongoing hemolysis. PNH appears to be a hematopoietic stem cell disorder that affects erythroid, granulocytic, and megakaryocytic cell lines. The abnormal cells in PNH have been shown to lack glycosylphosphatidylinositol (GPI)-linked proteins in RBCs and WBCs. Mutations in the phosphatidylinositol glycan A (PIGA) gene have been identified consistently in patients with PNH, thus confirming the biologic defect in this disorder.
Flow cytometric immunophenotyping of peripheral blood (WBC and RBC) is performed to determine the presence or absence of PI-linked antigens (CD14, FLAER, and/or CD59 antigens) using monoclonal antibodies directed against them. These proteins are absent on the cells of patients with PNH. Certain GPI-anchored proteins protect red blood cells from destruction; others are involved in blood clotting, whereas others are involved in fighting infection. Therefore the majority of the disease manifestations (i.e., hemolytic anemia, thrombosis, and infection) result from a deficiency of these GPI-anchored proteins.
Individuals without PNH have normal expression of all PI-linked antigens—CD14 (monocytes), CD16 (neutrophils and NK cells), CD24 (neutrophils), and CD59 (RBCs). Other GPI-linked antigens noted to be absent in PNH include CD55, and CD59. In addition, FLAER, a fluorescently labeled inactive variant of aerolysin, binds directly to the GPI anchor and can be used to evaluate the expression of the GPI linkage.
Plasminogen (Fibrinolysin)
Indications
This test is used to diagnose suspected plasminogen deficiency in patients who present with multiple thromboembolic episodes.
Test Explanation
Plasminogen is a protein involved in the fibrinolytic process of intravascular blood clot dissolution (see Figure 2-12 on p. 167). Plasminogen is converted to plasmin by proteolytic cleavage. This reaction can be catalyzed by urokinase, streptokinase, or tissue plasminogen activator (t-PA). Plasmin can destroy fibrin and dissolve clots. This fibrinolytic system helps maintain a normal homeostatic balance between coagulation and anticoagulation.
Plasminogen levels are occasionally measured during fibrinolytic therapy (for coronary and peripheral arterial occlusion) and are diminished. Decreased levels of plasminogen are also found in hyperfibrinolytic states (e.g., disseminated intravascular coagulation [DIC], primary fibrinolysis), because the plasminogen is used up. Because plasminogen is made in the liver, patients with cirrhosis or other severe liver diseases can be expected to have decreased levels. There are also rare cases of hereditary deficiencies of this protein. Decreased plasminogen levels put a patient at great risk for arterial or venous thrombosis.
Pregnancy and especially eclampsia are associated with increased levels of plasminogen. Patients with inflammatory conditions that may be associated with increased levels of C-reactive protein also may have concomitant mild elevations of plasminogens, which are acute-phase reactant proteins.
Test Results and Clinical Significance
Decreased Levels
Hyperfibrinolytic state (e.g., DIC, fibrinolysis)
Primary liver disease: Plasminogen is made in the liver. With severe liver disease, synthesis will not occur.
Syndrome associated with hypercoagulation (e.g., venous and arterial clotting): This syndrome can occur with many different types of diseases (e.g., colon cancer).
Congenital deficiencies of plasminogen: Although rare, such deficiencies can occur and place the patient at great risk for thromboembolic episodes.
Malnutrition: With severe malnutrition, protein depletion is so great that it interrupts plasminogen production.
Plasminogen Activator Inhibitor 1 Antigen/Activity (PAI-1)
Indications
Plasminogen activator inhibitor 1 (PAI-1) is the principal inactivator of the fibrinolytic system. High levels are associated with a number of atherosclerotic risk factors.
Test Explanation
PAI-1 is a protein that inhibits plasminogen activators. During fibrinolysis, tissue plasminogen activator (tPA) converts plasminogen into plasmin. Plasmin plays a critical role in fibrinolysis by degrading fibrin (see Figure 2-12 on p. 167). PAI-1 is the primary inhibitor of tPA and urokinase plasminogen activator (uPA) in the blood. PAI-1 limits the production of plasmin and keeps fibrinolysis in check.
Elevated levels of PAI-1 are associated with a predisposition to thrombosis, including veno-occlusive disease after bone marrow transplantation or high-dose chemotherapy. Familial thrombosis has been associated with inherited elevation of plasma PAI-1 activity. Increased levels of PAI-1 have also been reported in a number of conditions including malignancy, liver disease, the postoperative period, septic shock, the second and third trimesters of pregnancy, obesity, coronary heart disease, and in patients with restenosis after coronary angioplasty. Increased levels may reduce the effectiveness of antithrombolytic therapy. Patients with insulin resistance syndrome and diabetes mellitus tend to have increased PAI-1 levels.
Low plasma levels of the active form of PAI-1 have been associated with abnormal clinically significant bleeding. Complete deficiency of PAI-1, either congenital or acquired, is associated with bleeding manifestations that include hemarthroses, hematomas, menorrhagia, easy bruising, and postoperative hemorrhage. Most laboratory tests are not capable of accurately quantifying low concentrations of PAI-1; therefore PAI-1 deficiency is difficult to identify.
PAI-1 is an acute phase reactant, and it will fluctuate in the face of an acute infection. Furthermore there is a top normal diurnal variation associated with PAI-1 levels. PAI-1 antigen can be measured directly by ELISA or by PCR genotyping. PAI-1 activity can be measured by bio-immunoassay. Results vary by methodology.
Test Results and Clinical Significance
Related Tests
Plasminogen (p. 394). This test is used to diagnose suspected plasminogen deficiency in patients who present with multiple thromboembolic episodes.
Protein S/C (p. 432). This test identifies patients who are deficient in protein C and/or S. This is part of the evaluation of patients with coagulation disorders.
Factor V-Leiden (p. 231). This test is used to diagnose factor V-Leiden thrombophilia and is used in the evaluation of increased thrombosis.
Platelet Aggregation
Indications
This test is a measure of platelet function and aids in the evaluation of bleeding disorders.
Test Explanation
Platelet aggregation is important in hemostasis. A clump of platelets surrounds an area of acute blood vessel endothelial injury. Normal platelets adhere to this area of injury, and through a series of chemical reactions, they attract other platelets to the area. This is platelet aggregation, the first step in hemostasis. After this step the normal coagulation factor cascade occurs. Certain diseases that affect either platelet number or function can inhibit platelet aggregation and thereby prolong bleeding times. Congenital syndromes, uremia, myeloproliferative disorders, and drugs are associated with abnormal platelet aggregation. If blood is passed through a heart-lung or dialysis pump, platelet injury can occur and aggregation capability can be reduced.
Many agonists are used to stimulate platelet aggregation in the laboratory. Platelet aggregation is measured by determining the turbidity of platelet-rich plasma. As platelet aggregation is stimulated in vitro, turbidity decreases and light transmission through the specimen increases. This test is performed with an optical device called an aggregometer. Usually the patient's blood specimen is spun to a platelet-rich component. Next, an agonist to platelet aggregation, such as adenosine diphosphate, collagen, epinephrine, or ristocetin, is added. Turbidity is then measured within the aggregometer, and a curve indicating light transmission per unit of time is plotted. Normal curves have been identified for any one agonist that is used.
This is a very sensitive test, and it can be significantly affected by a number of variables, including:
Interfering Factors
• Factors that may cause increased platelet aggregation include blood storage temperature, hyperbilirubinemia, hemoglobinemia, hyperlipidemia, and platelet count.
Drugs that may cause decreased platelet aggregation include aspirin, antibiotics, nonsteroidal antiinflammatory agents, and thienopyridine antiplatelet drugs, such as ticlopidine (Ticlid) and clopidogrel (Plavix).
Test Results and Clinical Significance
Prolonged Platelet Aggregation
Various congenital disorders (e.g., Wiskott-Aldrich syndrome, Bernard-Soulier syndrome, von Willebrand disease): Platelet aggregation is diminished in autosomal recessive diseases.
Connective tissue disorder (e.g., lupus erythematosus): The pathophysiology of these observations is not understood.
Recent cardiopulmonary or dialysis bypass: Platelet injury develops as the platelets are passing through this machinery. The injured platelets are less likely to function normally in regard to aggregation.
Uremia: Not only is there a reduced platelet number in uremic patients, but a reduced aggregation capability has also been observed.
Various myeloproliferative diseases, including leukemia, myeloma, and dysproteinemia: The pathophysiology of these observations is not clear. It may be related to abnormal antibodies affecting the platelet membrane.
Drugs (e.g., aspirin): Drugs can have an immediate, and in some cases long-lasting, negative effect on platelet aggregation.
Related Tests
Platelet Count (p. 401). This is a direct measurement of platelet number.
Platelet Antibody (see following test). This test identifies antibodies directed against platelets.
Platelet Volume, Mean (p. 407). This is a measurement of the size of the platelets. It is helpful in the evaluation of thrombocytopenia.
Platelet Antibody (Antiplatelet Antibody Detection)
Indications
This test is used to evaluate thrombocytopenia and exclude an immune-associated etiology.
Test Explanation
Immune-mediated destruction of platelets may be caused by either autoantibodies directed against antigens located on the same person's platelets or alloantibodies that develop after exposure to transfused platelets received from a donor. These antibodies are usually directed to an antigen on the platelet membrane, such as human leukocyte antigen (HLA) (see p. 306) or platelet-specific antigen (e.g., PLA1, PLA2). Many different laboratory techniques can be used to demonstrate the antiplatelet antibodies. These tests can directly identify the immunoglobulin with the use of radioimmunoassay (RIA) or immunofluorescence. Quantitative measurements are possible with cytofluorometry. Other tests identify complement binding on the affected platelet membrane. Most antiplatelet antibody testing is now performed using immunologic assays.
Antibodies directed to platelets will cause early destruction of the platelets and subsequent thrombocytopenia. Immunologic thrombocytopenia includes the following:
1. Idiopathic thrombocytopenia purpura (ITP) is a term that describes a group of disorders characterized by immune-mediated destruction of the platelets within the spleen or other reticuloendothelial organs. Platelet-associated immunoglobulin (Ig)G antibodies are detected in 90% of these patients.
2. Posttransfusion purpura is a rare syndrome characterized by the sudden onset of severe thrombocytopenia a few hours to a few days after transfusion of red blood cells (RBCs) or platelets. This is usually associated with an antibody to AB, B, and O (ABO), HLA, or PLA antigens on the RBC. In most situations the blood recipient has previously been sensitized to a PLA1 antigen during previous transfusions or during previous pregnancy. Once these antibodies form, they destroy the donor's PLA1-positive platelets and the recipient's PLA-negative platelets.
3. Maternal-fetal platelet antigen incompatibility (neonatal thrombocytopenia) occurs when the fetal platelet contains a PLA1 antigen that is absent in the mother. Just like Rh RBC incompatibility, the mother creates anti-PLA1 antibodies that cross the placenta and destroy the fetal platelets. The mother is not thrombocytopenic. Neonatal thrombocytopenia can also occur if the mother has ITP autoantibodies that are passed through the placenta and destroy the fetal platelets.
4. Drug-induced thrombocytopenia. While a host of drugs are known to induce autoimmune-mediated thrombocytopenia, heparin is the most common and causes heparin-induced thrombocytopenia (HIT). There are two types of HIT, type I and II, that may develop. Type I HIT is generally considered a benign condition and is not antibody mediated. In type II HIT, thrombocytopenia is usually more severe and is antibody mediated. Type II HIT is caused by an IgG antibody and usually occurs after 6 to 8 days of intravenous heparin therapy. Although platelet counts may be low, bleeding is unusual. Rather, paradoxic thromboembolism is the most worrisome complication and may be attributable to platelet activation caused by the anti–H-PF4 antibody complex instigating platelet aggregation.
HIT occurs in about 1% to 5% of patients taking heparin for 5 to 10 days, and heparin-induced thrombosis occurs in one third to one half of these patients. Cessation of heparin is mandatory, and alternative anticoagulation is initiated. The diagnosis is suspected based on clinical symptoms, recent heparin administration, and low platelet counts. The diagnosis is confirmed by identifying heparin-induced thrombocytopenia antibodies (HITA). This test uses an enzyme-linked immunosorbent assay to detect HIT-specific antibodies to heparin-PF4 complex. This assay can detect IgG, IgM, and IgA antibodies, and has a sensitivity of approximately 80% to 90%.
Other drugs known to cause antiplatelet antibodies include cimetidine, analgesics (salicylates, acetaminophen), antibiotics (cephalosporins, penicillin derivatives, sulfonamides), quinidine-like drugs, diuretics (e.g., chlorothiazide), and others (e.g., digoxin, propylthiouracil, disulfiram [Antabuse]).
Procedure and Patient Care
After
• Apply pressure or a pressure dressing to the venipuncture site.
• Ensure adequate hemostasis in all patients with suspected thrombocytopenia.
• A platelet count is usually done 1 to 2 hours after platelet transfusion. This not only documents the posttransfusion platelet count, but it also eliminates a large proportion of posttransfusion immune thrombocytopenia reactions.
Platelet Count (Thrombocyte Count)
Indications
The platelet count is an actual count of the number of platelets (thrombocytes) per cubic milliliter of blood. It is performed on patients who develop petechiae (small hemorrhages in the skin), spontaneous bleeding, increasingly heavy menses, or thrombocytopenia. It is used to monitor the course of the disease or therapy for thrombocytopenia or bone marrow failure.
Test Explanation
Platelets are formed in the bone marrow from megakaryocytes. They are small, round, nonnucleated cells whose main role is maintenance of vascular integrity. In blood vessel injury, hemostasis is required to form a clot that will durably plug the hole until healing can occur. The primary phase of the hemostatic mechanism involves platelet aggregation. From there, the platelets help initiate the coagulation factor cascade. Most of the platelets exist in the bloodstream. A smaller percentage (25%) exists in the liver and spleen. Survival of platelets is measured in days (average of 7 to 9 days).
Platelet activity is essential to blood clotting. Counts of 150,000 to 400,000/mm3 are typically considered normal. Counts of less than 100,000/mm3 are generally considered to indicate thrombocytopenia; thrombocytosis (thrombocythemia) is generally said to exist when counts are greater than 400,000/mm3. Vascular thrombosis with tissue or organ infarction is the major complication of thrombocythemia. Common diseases associated with spontaneous thrombocytosis are iron deficiency anemia and malignancy (leukemia, lymphoma, solid tumors such as of the colon). Thrombocytosis may also occur with polycythemia vera, postsplenectomy syndromes, and a variety of acute/chronic infections or inflammatory processes. It should be noted that even patients with elevated platelet counts can experience a bleeding tendency because the function (platelet aggregation) of those platelets may be abnormal.
Spontaneous hemorrhage may occur with thrombocytopenia. If thrombocytopenia is severe, the platelets are often hand counted. Spontaneous bleeding is a serious danger when platelet counts fall below 20,000/mm3. Petechiae and ecchymosis will also occur at that degree of thrombocytopenia. With counts above 40,000/mm3, spontaneous bleeding rarely occurs, but prolonged bleeding from trauma or surgery may occur with counts at this level.
Causes of thrombocytopenia include:
1. Reduced production of platelets (secondary to bone marrow failure or infiltration of fibrosis, tumor, etc.)
2. Sequestration of platelets (secondary to hypersplenism)
3. Accelerated destruction of platelets (secondary to antibodies, infections, drugs, prosthetic heart valves)
4. Consumption of platelets (secondary to disseminated intravascular coagulation [DIC])
5. Platelet loss from hemorrhage
6. Dilution with large volumes of blood transfusions containing very few, if any, platelets
Interfering Factors
• Living in high altitudes may cause increased platelet levels.
• Because platelets can clump together, automated counting is subject to at least a 10% to 15% error.
• Strenuous exercise may cause increased levels.
• Decreased levels may be seen before menstruation.
Drugs that may cause increased levels include estrogens and oral contraceptives.
Drugs that may cause decreased levels include chemotherapeutic agents, chloramphenicol, colchicine, histamine-2–(H2)-blocking agents (cimetidine, Zantac), hydralazine, indomethacin, isoniazid (INH), quinidine, streptomycin, sulfonamides, thiazide diuretics, and tolbutamide (Orinase).
Procedure and Patient Care
Test Results and Clinical Significance
Increased Levels (Thrombocytosis)
Malignant disorders (leukemia, lymphoma, solid tumors such as of the colon): The pathophysiology of this observation is not known.
Polycythemia vera: This is a hyperplasia of all the marrow cell lines, including platelets.
Postsplenectomy syndrome: The spleen normally extracts aging platelets from the bloodstream. With surgical splenectomy, that job is less effectively done by other organs (liver, etc.). As a result, the platelet count increases.
Rheumatoid arthritis: The pathophysiology of this observation is not known.
Iron-deficiency anemia or following hemorrhagic anemia: Iron is not needed for platelet production. Anemia causes maximal stimulation of cellular production by the marrow. Red blood cells (RBCs) may not be so easily produced in light of iron deficiency. The platelet, however, can easily respond even in the presence of iron deficiency.
Decreased Levels (Thrombocytopenia)
Hypersplenism: The spleen normally extracts aging platelets from the bloodstream. An enlarged spleen, however, extracts more platelets, both aging and new. The platelet count diminishes.
Hemorrhage: The platelets are lost in the bleeding process. If not replaced by transfusion of platelets, it will take some time (hours to days) for the marrow to produce an adequate number of platelets. This problem is exacerbated with treatment that replenishes blood volume and RBC count. This treatment dilutes the remaining platelets and further decreases the platelet count.
Immune thrombocytopenia (e.g., idiopathic thrombocytopenia, neonatal, posttransfusion, or drug-induced thrombocytopenia): Antibodies directed against antigens on the platelet cell membrane destroy the platelet and the count decreases.
Leukemia and other myelofibrosis disorders: The marrow is replaced by neoplastic or fibrotic tissue. Megakaryocyte function and numbers diminish. Platelets are not produced, and the count drops.
Thrombotic thrombocytopenia: This disease and others such as HELLP (hemolysis [H], elevated liver enzymes [EL], low platelet count [LP]) syndrome are highlighted by thrombocytopenia, hemolytic anemia, and other hematologic abnormalities.
Graves disease: In a small number of these patients, thrombocytopenia occurs. The pathophysiology of this observation is not known.
Inherited disorders (e.g., Wiskott-Aldrich, Bernard-Soulier, Zieve syndromes): The pathophysiology of this observation is not known.
DIC: The pathophysiology of thrombocytopenia is not clear. In part, however, it is thought that ongoing thrombosis “consumes” the platelets much like coagulating factors are “consumed.” DIC usually develops concurrently with other severe disease (e.g., gram-negative sepsis) that can also produce thrombocytopenia.
Systemic lupus erythematosus: The pathophysiology of this observation is not known.
Pernicious anemia: Unlike iron, vitamin B12 is necessary for platelet production. A deficiency of this vitamin or folate will diminish the production of platelets.
Some hemolytic anemias: Often the same disease process that produces the hemolysis (e.g., hemolytic-uremic syndrome) also destroys the platelets. The platelet count falls.
Cancer chemotherapy: Cytotoxic drugs often affect the bone marrow. Platelets are not produced at adequate levels, and the count drops.
Acute/chronic infections: Bacterial, viral, and rickettsial infections can cause thrombocytopenia, especially when the patient is immunocompromised (e.g., acquired immunodeficiency syndrome [AIDS]).
Related Tests
Platelet Aggregation (p. 398). This is a test of platelet function.
Platelet Antibody (p. 399). This test identifies antibodies directed against platelets.
Platelet Function Assay (see following test). This test is used to identify platelet dysfunction in patients with normal platelet counts with prolonged bleeding.
Platelet Volume, Mean (p. 407). This is a measurement of the size of the platelets. It is helpful in the evaluation of thrombocytopenia.
Platelet Function Assay (Platelet Closure Time, PCT, Aspirin Resistance Tests)
Indications
This test is used to identify platelet dysfunction in patients who are suspected of having a bleeding abnormality. This test can identify abnormalities in the ability of platelets to aggregate or instigate the hemostatic cascade. It is used for patients with a family or personal history of acute excessive bleeding.
Test Explanation
Platelet dysfunction may be acquired, inherited, or induced by platelet-inhibiting agents. It is clinically important to assess platelet function as a potential cause of a bleeding diathesis (epistaxis, menorrhagia, postoperative bleeding, or easy bruising). The most common causes of platelet dysfunction are related to uremia, liver disease, von Willebrand disease (vWD), and exposure to such agents as acetyl salicylic acid (ASA, aspirin). The bleeding time (BT) test was a commonly performed test to evaluate platelet function. BT is labor intensive and expensive, and its accuracy is heavily dependent on operator skills. Its results cannot be reproduced and quantified. Platelet aggregation studies (p. 398) have similar accuracy problems. As a result, more clinical laboratories are using the platelet closure time (PCT) to more accurately quantify platelet function. Furthermore PCT can differentiate aspirin affects from other causes of platelet dysfunction (Figure 2-20).
Figure 2-20 Dade Behring automated platelet function analyzer capable of determining platelet hemostatic function and the effect of medications on that function.
Closure times are performed on a system in which the process of platelet adhesion and aggregation following a vascular injury is simulated in vitro. Anticoagulated whole blood is passed over membranes at a standardized flow rate, creating high shear rates that result in platelet attachment, activation, and aggregation on the membrane. A hole in the membrane is occluded when a stable platelet plug develops. The time required to obtain full occlusion of the aperture is reported as the PCT in seconds. The test is sensitive to platelet adherence and aggregation abnormalities, and allows the discrimination of aspirin-like defects and intrinsic platelet disorder. If a collagen/epinephrine (CEPI) CEPI membrane is used during testing, intrinsic platelet dysfunction can be identified. If a collagen/adenosine-5′-diphosphate (CADP) membrane is used during testing, the impact of aspirin on platelets can be determined. This test also can be used to determine resistance of aspirin's therapeutic anticoagulation effects on platelets. This is one of several aspirin resistance tests that are performed to determine the effectiveness of aspirin on inhibiting platelet aggregation and thereby protecting patient from vascular thromboembolic disease (Table 2-39).
Another aspirin resistance test is measurement of 11-dehydro-thromboxane B2 (11-dTXB2) in the urine. Thromboxane A2 is produced by the enzyme cyclo-oxygenase-1 (COX1) by activated platelets and still further stimulates platelet activation, platelet aggregation, and vasoconstriction. 11-Dehydro-thromboxane B2 (11-dTXB2) is the stable, inactive metabolite of thromboxane A2. Urinary 11-dTXB2 therefore is an indication of platelet activation and aggregation. Elevated values are associated with increased risk of acute ischemic stroke and myocardial infarction. Effective aspirin therapy should reduce the level of this metabolite in the urine. If not, the patient may be aspirin resistant and may be more safely treated with an alternative therapy, including increasing the dosage of aspirin or placing the patient on another antiplatelet medication. A positive test for aspirin resistance raises the possibility that the patient may be clopidogrel (Plavix) resistant as well.
Urinary 11-dTXB2 offers an advantage over blood aspirin resistance tests because it is not subject to interference from in vitro platelet activation caused by local vein trauma or insufficient anticoagulation during blood sample collection.
Interfering Factors
• Low hematocrit or platelet count can decrease PCT.
Aspirin and nonsteroidal antiarthritic agents (NSAIDS) can increase test results. These medications prevent blood from clotting by blocking the production of thromboxane A2, a chemical that platelets produce that instigates platelet aggregation. Aspirin accomplishes this by inhibiting the enzyme cyclooxygenase-1 (COX-1) that produces thromboxane A2.
Thienopyridines can increase test results. When ADP attaches to ADP receptors on the surface of platelets, the platelets clump. The thienopyridines (e.g., ticlopidine [Ticlid] and clopidogrel [Plavix]) block the ADP receptor, which prevents ADP from attaching to the receptor and the platelets from clumping.
Test Results and Clinical Significance
Related Test
Platelet Count (p. 401). This is a quantitative measure of the number of circulating platelets. Thrombocytopenia is a common cause of excessive bleeding.
Platelet Volume, Mean (Mean Platelet Volume [MPV])
Indications
This test is helpful in the evaluation of platelet disorders, especially thrombocytopenia.
Test Explanation
The MPV is a measure of the volume of a large number of platelets determined by an automated analyzer. MPV is to platelets as mean corpuscular volume (see p. 442) is to the red blood cells (RBCs).
The MPV varies with total platelet production. In cases of thrombocytopenia despite a normal reactive bone marrow (e.g., hypersplenism), the normal bone marrow releases immature platelets in an attempt to maintain a normal platelet count. These immature platelets are larger, and the MPV is increased. When bone marrow production of platelets is inadequate, the platelets that are released are small. This will be reflected as a low MPV; this makes the MPV useful in the differential diagnosis of thrombocytopenic disorders. However, because of variable results, the use of the MPV has decreased.
Procedure and Patient Care
Test Results and Clinical Significance
Increased Levels
Immune thrombocytopenia (e.g., idiopathic thrombocytopenia, neonatal, posttransfusion, or drug induced-thrombocytopenia),
The above illnesses are all associated with thrombocytopenia and a normally reactive bone marrow that will produce a great number of immature platelets in an attempt to maintain a normal platelet count. These immature platelets are large and increase the MPV.
Vitamin B12 or folate deficiency: Megaloblastic changes affect the megakaryocyte just as the erythroid line is affected. The platelets that are produced are larger and may even be nucleated. The MPV is increased.
Myelogenous leukemia: Large, abnormal platelets are formed by neoplastic megakaryocytes if they are involved in the leukemic process. The MPV will increase.
Decreased Levels
Chemotherapy-induced myelosuppression:
When bone marrow production of platelets is inadequate, the platelets that are released are small. MPV will be reduced.
Wiskott-Aldrich syndrome: This syndrome is characterized by eczema, immune deficiency, thrombocytopenia, and small platelets.
Potassium, Blood (K)
Indications
This test is routinely performed in most patients evaluated for any type of serious illness. Furthermore, because this electrolyte is so important to cardiac function, it is a part of all complete routine evaluations, especially in patients who take diuretics or heart medications.
Test Explanation
Potassium is the major cation within the cell. The intracellular potassium concentration is approximately 150 mEq/L, whereas the normal serum potassium concentration is approximately 4 mEq/L. This ratio is the most important determinant in maintaining membrane electrical potential, especially in neuromuscular tissue. Because the serum concentration of potassium is so small, minor changes in concentration have significant consequences. Potassium is excreted by the kidneys. There is no reabsorption of potassium from the kidneys. Therefore, if potassium is not adequately supplied in the diet (or by intravenous [IV] administration in the patient who is unable to eat), serum potassium levels can drop rapidly.
Potassium is an important part of protein synthesis and maintenance of normal oncotic pressure and cellular electrical neutrality as indicated above. It contributes to the metabolic portion of acid-base balance in that the kidneys can shift potassium for hydrogen ions to maintain a physiologic pH.
Serum potassium concentration depends on many factors, including:
1. Aldosterone (and, to a lesser extent, glucocorticosteroids). This hormone tends to increase renal losses of potassium.
2. Sodium reabsorption. As sodium is reabsorbed, potassium is lost.
3. Acid-base balance. Alkalotic states tend to lower serum potassium levels by causing a shift of potassium into the cell. Acidotic states tend to raise serum potassium levels by reversing that shift.
Symptoms of hyperkalemia include irritability, nausea, vomiting, intestinal colic, and diarrhea. The electrocardiogram may demonstrate peaked T waves, a widened QRS complex, and a depressed ST segment. Signs of hypokalemia are related to a decrease in contractility of smooth, skeletal, and cardiac muscles, which results in weakness, paralysis, hyporeflexia, ileus, increased cardiac sensitivity to digoxin, cardiac arrhythmias (dysrhythmias), flattened T waves, and prominent U waves. This electrolyte has profound effects on the heart rate and contractility. The potassium level should be carefully followed in patients with uremia, Addison disease, and vomiting and diarrhea and in patients taking steroid therapy and potassium-depleting diuretics. Potassium must be closely monitored in patients taking digitalis-like drugs, because cardiac arrhythmias may be induced by hypokalemia and digoxin.
Interfering Factors
• Opening and closing of the hand with a tourniquet in place may increase potassium levels.
• Hemolysis of blood during venipuncture or during laboratory processing causes increased levels.
Drugs that may cause increased potassium levels include aminocaproic acid, antibiotics, antineoplastic drugs, captopril, epinephrine, heparin, histamine, isoniazid (INH), lithium, mannitol, potassium-sparing diuretics, potassium supplements, and succinylcholine.
Drugs that may cause decreased levels include acetazolamide, aminosalicylic acid, glucose infusions, amphotericin B, carbenicillin, cisplatin, diuretics (potassium wasting), insulin, laxatives, lithium carbonate, penicillin G sodium (high doses), phenothiazines, salicylates (aspirin), and sodium polystyrene sulfonate (Kayexalate).
Clinical Priorities
• This electrolyte has profound effects on the heart rate and contractility. Potassium levels must be carefully monitored in patients taking digitalis-like drugs and diuretics, because cardiac arrhythmias may be induced by hypokalemia.
• Intravenous potassium may be indicated to prevent cardiac arrhythmias for hypokalemia in the adult. Potassium is infused at a slow rate to prevent irritation to the veins.
• Serum potassium levels are affected by acid-base balance. Alkalotic states lower potassium levels and acidotic states raise levels.
• Hemolysis of blood during venipuncture or laboratory processing can cause elevations.
Procedure and Patient Care
After
• Apply pressure or a pressure dressing to the venipuncture site.
• Assess the venipuncture site for bleeding.
• Evaluate the patient with increased or decreased potassium levels for cardiac arrhythmias.
• Monitor patients taking digoxin and diuretics for hypokalemia.
• If indicated, administer resin exchanges (e.g., Kayexalate enema) to correct hyperkalemia.
Test Results and Clinical Significance
Increased Levels (Hyperkalemia)
Because the amount of potassium in the serum is so small, minimal but significant increases in potassium intake can cause elevations in the serum level.
Acute or chronic renal failure: This is the most common cause of hyperkalemia. Potassium excretion is diminished, and potassium levels rise.
Aldosterone-inhibiting diuretics (e.g., spironolactone, triamterene):
Aldosterone excretion is absent. Aldosterone enhances potassium excretion. Without that effect, potassium excretion is diminished and potassium levels rise.
Transfusion of hemolyzed blood,
Potassium exists in high levels in the cell. With cellular injury and lysis, the potassium within the cell is released into the bloodstream.
Acidosis: To maintain physiologic pH during acidosis, hydrogen ions are driven from the blood and into the cell. To maintain electrical neutrality, potassium is expelled from the cell. Potassium levels rise.
Dehydration: The potassium becomes more concentrated in dehydrated patients, and serum levels appear to be elevated. When the patient is rehydrated, potassium levels may in fact be reduced.
Decreased Levels (Hypokalemia)
The kidneys cannot reabsorb potassium to compensate for the reduced potassium intake. Potassium levels decline.
Gastrointestinal (GI) disorders (e.g., diarrhea, vomiting, villous adenomas):
Diuretics: These medications act to increase renal excretion of potassium. This is especially important for cardiac patients who take diuretics and digitalis preparations. Hypokalemia can exacerbate the ectopy that digoxin may instigate.
Hyperaldosteronism: Aldosterone enhances potassium excretion.
Cushing syndrome: Glucocorticosteroids have an “aldosterone-like” effect.
Renal tubular acidosis: Renal excretion of potassium is increased.
Licorice ingestion: Licorice has an “aldosterone-like” effect.
Alkalosis: To maintain physiologic pH during alkalosis, hydrogen ions are driven out of the cell and into the blood. To maintain electrical neutrality, potassium is driven into the cell. Potassium levels fall.
Insulin administration: In patients with hyperglycemia, insulin is administered. Glucose and potassium are driven into the cell. Potassium levels drop.
Glucose administration: In a normal person, insulin is secreted in response to glucose administration. Glucose and potassium are driven into the cell. Potassium levels drop.
Ascites: These patients have a decreased renal blood flow from reduced intravascular volume that results from the collection of fluid. The reduced blood flow stimulates the secretion of aldosterone, which increases potassium excretion. Furthermore, these patients are often taking potassium-wasting diuretics.
Renal artery stenosis: These patients have a reduced renal blood flow. The pathophysiology is as described above.
Cystic fibrosis: These patients have increased potassium loss in secretions and sweat.
Trauma/surgery/burns: The body's response to trauma is mediated, in part, by aldosterone, which increases potassium excretion.
Prealbumin (PAB, Thyroxine-Binding Prealbumin [TBPA], Thyretin, Transthyretin)
Indications
This test is used to indicate a person's nutritional status. It is also used to indicate liver function status.
Test Explanation
Prealbumin is one of the major plasma proteins. Because prealbumin can bind thyroxine, it is also called thyroxine-binding prealbumin. However, prealbumin is secondary to thyroxine-binding globulin in the transportation of triiodothyronine (T3) and thyroxine (T4). Prealbumin also plays a role in the transport and metabolism of vitamin A. Prealbumin is measured by immunoassay.
Because prealbumin levels in serum fluctuate more rapidly in response to alterations in synthetic rate than do those of other serum proteins, clinical interest in the quantification of serum prealbumin has centered on its usefulness as a marker of nutritional status. Its half-life of 1.9 days is much less than the 21-day half-life of albumin (see p. 424). Because of prealbumin's short half-life, it is a sensitive indicator of any change affecting protein synthesis and catabolism. Therefore prealbumin is frequently ordered to monitor the effectiveness of total parenteral nutrition (TPN).
Prealbumin is significantly reduced in hepatobiliary disease because of impaired synthesis. Serum levels of prealbumin are better indicators of liver function than albumin levels. Prealbumin is also a negative acute-phase reactant protein; serum levels decrease in inflammation, malignancy, and protein-wasting diseases of the intestines or kidneys. Because zinc is required for synthesis of prealbumin, low levels occur with zinc deficiency. Increased levels of prealbumin occur in Hodgkin disease and chronic kidney disease.
Because of the low quantity of prealbumin in the serum, this protein is not often visualized on serum protein electrophoresis. However, because prealbumin crosses the blood-brain barrier, it is found in the CSF and can be seen on CSF electrophoresis (see discussion of lumbar puncture on p. 651).
Interfering Factors
• Coexistent inflammation may make test result interpretation impossible.
Drugs that may cause increased levels include anabolic steroids, androgens, estrogen, and prednisolone.
Drugs that may cause decreased levels include amiodarone, estrogens, and oral contraceptives.
Clinical Priorities
• Clinical interest in prealbumin has centered on its usefulness as a marker of nutritional status. Its half-life of 1.9 days is much less than the 21-day half-life of albumin.
• Prealbumin is frequently indicated to monitor the effectiveness of TPN.
• Because prealbumin is a negative acute-phase reactant protein, serum levels may decrease with inflammatory processes and may make test result interpretation impossible.
Test Results and Clinical Significance
Increased Levels
Some cases of nephrotic syndrome: The major characteristic of the nephrotic syndrome is proteinuria that causes hypoproteinemia. Because prealbumin is so rapidly made, a disproportionate percentage of prealbumin can exist in the blood when other proteins take somewhat longer to produce.
Hodgkin disease: The pathophysiology of this observation is not known.
Pregnancy: The estrogen effect stimulates protein (prealbumin) synthesis.
Related Tests
Protein (p. 424). This is a quantification of all the components that make up the serum proteins, including albumin, alpha1, alpha2, beta1, beta2, and gamma globulins. This test can also detect abnormal proteins created by neoplasms and/or infections.
Immunoglobulin Quantification (p. 312). This is a quantification of the components that make up immunoglobulin, which is a gamma globulin.
Pregnancy-Associated Plasma Protein-A (PAPP-A)
Test Explanation
Pregnancy-associated plasma protein-A (PAPP-A) is made by the trophoblasts during pregnancy and released into the maternal circulation during pregnancy. Women with low blood levels of PAPP-A at 8 to 14 weeks of gestation have an increased risk of intrauterine growth restriction, trisomy 18 or 21, premature delivery, preeclampsia, and stillbirth. This protein rapidly rises in the first trimester of normal pregnancy. However, in Down-affected pregnancy, serum levels are half that of unaffected pregnancies. Furthermore, low first-trimester levels of PAPP-A in maternal serum are associated with adverse fetal outcomes, including fetal death in utero and intrauterine growth retardation.
This test is commonly used in conjunction with other pregnancy/maternal screening test (p. 354). Most first-trimester maternal screens include nuchal translucency (p. 888) measurement (a sonographic marker shown to be effective in screening fetuses for Down syndrome) and a blood draw analyte such as human chorionic gonadotropin (p. 304) or PAPP-A. A mathematical model is used to calculate a risk estimate by combining the analyte values, NT measurement, and maternal demographic information. The laboratory establishes a specific cutoff for each condition, which classifies each screen as either screen-positive or -negative.
A screen-negative result indicates that the calculated screen risk is below the established cutoff of 1:230 for Down syndrome and 1:100 for trisomy 18. A negative screen does not guarantee the absence of trisomy 18 or Down syndrome. Screen-negative results typically do not warrant further evaluation. When a Down syndrome risk cutoff of 1:230 is used for follow-up, the combination of maternal age, pregnancy-associated plasma protein A, human chorionic gonadotropin, and nuchal translucency has an overall detection rate of approximately 85% with a false-positive rate of 5% to 10%. A screen-positive result indicates that the value obtained exceeds the established cutoff. A positive screen does not provide a diagnosis, but indicates that further evaluation should be considered.
PAPP-A is present in unstable atherosclerotic plaques, and circulating levels are elevated in acute coronary syndromes, which may reflect the instability of the plaques. PAPP-A is an independent marker of unstable angina and acute myocardial infarction (heart attack). It is also a risk factor in predicting death after an acute myocardial event.
PAPP-A exists in a bound (to eosinophil major basic protein [pro-MBP]) and free form. In general, the bound form is most accurately predictive of pregnancy outcome, whereas the free form is the most accurate predictor in coronary atherosclerotic disease.
Interfering Factors
• All serum markers are adjusted for maternal weight (to account for dilution effects in heavier mothers). The estimated risk calculations and screen results are dependent on accurate information for gestation, maternal age, and weight. Inaccurate information can lead to significant alterations in the estimated risk.
Procedure and Patient Care
After
• Provide the results to the patient (and other family members if the patient desires) during a personal consultation.
• Allow the patient to express her concerns if the results are positive.
• Assist the patient in scheduling and obtaining more accurate diagnostic testing if the results are positive.
Test Results and Clinical Significance
Positive screening tests (trisomy 21, trisomy 18, neural tube defects, abdominal wall defects): This is an indication of risk, not a diagnosis. This is a screening test only. Further diagnostic testing would be required if positive.
Coronary atherosclerotic disease: By observation, unstable coronary plaques are associated with elevated PAPP-A levels.
Progesterone Assay
Normal Findings∗
Progesterone Level (ng/dL†)
Child:
<9 years: <20
10-15 years: <20
Adult
Male: 10-50
Female
Follicular phase: <50
Luteal phase: 300-2500
Postmenopausal: <40
Pregnancy (trimester)
First: 725-4400
Second: 1950-8250
Third: 6500-22,900
∗Considerable variation according to method used and laboratory.
†Extraction/radioimmunoassay.
Indications
This test is used in the evaluation of women who are having difficulty becoming pregnant or maintaining a pregnancy. It is also used to monitor “high-risk” pregnancies.
Test Explanation
Progesterone acts primarily on the endometrium. It initiates the secretory phase of the endometrium in anticipation of implantation of a fertilized ovum. Normally progesterone is secreted by the ovarian corpus luteum following ovulation. In pregnancy, progesterone is produced by the corpus luteum for the first few weeks. After that the placenta begins to make progesterone. Both serum progesterone levels and the urine concentration of progesterone metabolites (pregnanediol) are significantly increased during the latter half of a normal ovulatory cycle. Progesterone levels provide information about the occurrence and timing of ovulation.
Because progesterone levels rise rapidly after ovulation, this study is useful in documenting whether ovulation has occurred and, if so, its exact time. This is very useful information in women who have difficulty becoming pregnant. A series of measurements can help define the day of ovulation. Plasma progesterone levels start to rise after ovulation along with luteinizing hormone (LH), and they continue to rise for approximately 6 to 10 days. The levels then fall and menses occurs. Blood samples drawn at days 8 and 21 of the menstrual cycle normally will show a large increase in progesterone levels in the latter specimen, indicating that ovulation has occurred. Serum progesterone levels can provide comparable information and are sometimes measured in lieu of endometrial biopsy (see p. 726) to determine the phase of the menstrual cycle.
During pregnancy, progesterone levels normally rise because of the placental production of progesterone. Repeated assays can be used to monitor the status of the placenta in cases of “high-risk” pregnancy. Hormone assay for progesterone is used today to monitor progesterone supplementation in patients with an inadequate luteal phase to maintain an early pregnancy.
Interfering Factors
• Recent use of radioisotopes may affect test results if testing is done by radioimmunoassay (RIA).
• Hemolysis caused by rough handling of the sample may affect test results.
Drugs that may interfere with test results include estrogen, clomiphene, and progesterone.
Clinical Priorities
• Progesterone levels provide information about the occurrence and timing of ovulation. This is useful information in women having difficulty becoming pregnant.
• Hormone assays for progesterone are used to monitor progesterone supplementation in women with an inadequate luteal phase to maintain an early pregnancy.
• During pregnancy, progesterone levels normally rise because of placental production of progesterone. Repeated assays can be used to monitor placental states in “high-risk” pregnancies. Decreasing values are seen when placental viability is threatened.
Test Results and Clinical Significance
Increased Levels
Ovulation: This occurs with the normal development of a corpus luteum, which makes progesterone.
Pregnancy: A healthy placenta produces progesterone to maintain the pregnancy.
Luteal cysts of ovary: The corpus luteum produces progesterone in the nonpregnant female and in the early stages of pregnancy. Cysts can also produce progesterone for prolonged periods of time.
Adrenal cortical hormones are secreted at increased rates. 17-Hydroxyprogesterone is a precursor of these cortical hormones.
Choriocarcinoma of ovary: This tumor produces progesterone.
Molar pregnancy: Hydatidiform mole can produce progesterone, although at lower levels than pregnancy.
Decreased Levels
All of the above obstetric emergencies are associated with decreased placental viability. Progesterone is made by the placenta during pregnancy. Decreasing values are seen when placental viability is threatened.
Ovarian neoplasm: Ovarian epithelial cancers can destroy the functional ovarian tissue. Progesterone levels may decrease.
Related Test
Pregnanediol (p. 944). Pregnanediol is a catabolic metabolite of progesterone that is excreted via the kidneys into the urine.
Prolactin Level (PRL)
Indications
Prolactin levels are used to diagnose and monitor prolactin-secreting pituitary adenomas.
Test Explanation
Prolactin is a hormone secreted by the anterior pituitary gland (adenohypophysis). In females, prolactin promotes lactation. Its role in males has not been demonstrated. Prolactin secretion is controlled by prolactin-inhibiting and prolactin-releasing factors secreted by the hypothalamus. Thyroid-releasing hormone (TRH) can also stimulate prolactin production. During sleep, prolactin levels increase twofold to threefold, attaining circulating levels equaling those of pregnant women. With breast stimulation, pregnancy, nursing, stress, or exercise, a surge of this hormone occurs. Prolactin is elevated in patients with prolactin-secreting pituitary acidophilic or chromophobic adenomas. To a lesser extent, moderately high prolactin levels have been observed in women with secondary amenorrhea (i.e., postpubertal), galactorrhea, primary hypothyroidism, polycystic ovary syndrome, and anorexia. Paraneoplastic tumors (e.g., lung cancer) may cause ectopic secretion of prolactin as well. In general, very high prolactin levels are more likely to be related to pituitary adenoma than to other causes.
The prolactin level is helpful for monitoring the disease activity of pituitary adenomas. Several prolactin stimulation tests (with TRH or chlorpromazine) and prolactin suppression tests (with levodopa) have been designed to help differentiate pituitary adenoma from some other causes of prolactin overproduction. Prolactin levels are used to evaluate functional and organic disease of the hypothalamus, primary hypothyroidism, section compression of the pituitary stalk, chest wall lesions, renal failure, and ectopic tumors.
Hyperprolactinemia often results in loss of libido; galactorrhea; oligomenorrhea or amenorrhea and infertility in premenopausal women; and loss of libido, impotence, infertility, and hypogonadism in men. Prolactin values that exceed the reference values may result from macroprolactin (prolactin bound to immunoglobulin). Macroprolactin blood levels should be evaluated if signs and symptoms of hyperprolactinemia are absent or pituitary imaging studies are not informative. Macroprolactin can be inversely computed by measuring the percent of manometric prolactin. If the percent of monomeric prolactin is less than 40% of the total, macroprolactinemia exists and the patient does not have true elevated prolactin levels.
Interfering Factors
• Stress from illness, trauma, surgery, or even the fear of a blood test can elevate prolactin levels. In patients who are fearful of venipuncture, it is best to place a saline lock and draw the blood specimen 2 hours later.
Drugs that may cause increased values include antipsychotic drugs (risperidone phenothiazines), antinausea/antiemetic drugs, serotonin reuptake (antidepressants of all classes), ergot derivatives, some illegal drugs (e.g., cannabis), oral contraceptives, reserpine, opiates, histamine antagonists, monoamine oxidase inhibitors, estrogens/progesterone, several antihypertensive drugs, anticonvulsants (valproic acid), anti-tuberculous medications, and antihistamines.
Drugs that may cause decreased values are clonidine, dopamine, ergot alkaloid derivatives, and levodopa.
Procedure and Patient Care
Before
Explain the procedure to the patient.
Tell the patient no fasting or special preparation is required.
Inform the patient that this blood sample should be drawn in the morning.
Test Results and Clinical Significance
Increased Levels
Galactorrhea: Voluminous galactorrhea can be caused by elevated prolactin levels. A small-volume nipple discharge is quite common and not pathologic unless it is bloody.
Amenorrhea: Patients who have had normal menses and then stop having menses may be found to have elevated prolactin levels. Many are subsequently found to have prolactin-secreting pituitary adenomas.
Prolactin-secreting pituitary tumor: Most of these are benign adenomas of the acidophilic type.
Infiltrative diseases of hypothalamus and pituitary stalk (e.g., granuloma, sarcoidosis),
Metastatic cancer of pituitary gland:
The pathologic destruction of the hypothalamus or pituitary can destroy the prolactin-inhibiting regulatory mechanisms.
Hypothyroidism: Patients with hypothyroidism because of thyroid failure have elevated TRH levels. TRH also stimulates prolactin production.
Paraneoplastic syndrome: These cancers are associated with ectopic production of prolactin.
Stress (e.g., anorexia nervosa, surgery, strenuous exercise, trauma, severe illness): The pathophysiology of these observations is not known.
Empty sella syndrome: These patients have a large sella turcica noted on x-ray films but do not have a pituitary adenoma, yet they often have elevated prolactin levels.
Polycystic ovary syndrome: The pathophysiology of this observation is not well known.
Renal failure: These patients probably have a reduced clearance of prolactin.
Decreased Levels
Pituitary apoplexy (Sheehan syndrome): Women who have severe hemorrhage after obstetric delivery experience circulatory collapse. Their pituitary glands become infarcted. Prolactin levels are diminished along with other pituitary hormones.
Pituitary destruction by tumor (craniopharyngioma): Any disease that destroys the pituitary gland will, of course, be associated with reduced prolactin levels.