Hematology is the study of blood and includes the morphologic appearance of cells, function of blood, and diseases of blood and blood-forming tissues. Blood is composed of plasma, red blood cells, white blood cells, and platelets.
Plasma is a straw-colored liquid that makes up 55 percent of the total volume of blood. Plasma functions include transporting nutrients to the tissues of the body and picking up wastes from the tissues. Plasma is composed of 91 percent water and 9 percent plasma proteins.
Erythrocytes (red blood cells) transport oxygen from the lungs to body tissues. The normal adult range is 4.2–6.3 million per mm3. Red blood cells are biconcave in shape and do not contain a nucleus. The biconcave shape provides a greater surface area for the exchange of substances. Hemoglobin is the important component of red blood cells, which binds with and transports oxygen.
Leukocytes (white blood cells) are clear, colorless cells that contain a nucleus. These cells help protect the body from infections. White blood cells attempt to destroy invading pathogens and remove them from the body. The average white blood cell count is 4,500–11,000 per mm3. A level above 11,000 may be indicative of an infection or disease.
Platelets (clotting cells) or thrombocytes are small, clear cells that lack a nucleus. Platelets are formed in the red bone marrow from giant cells known as megakaryocytes. These cells also produce thrombokinase, an enzyme utilized in the clotting process. The average platelet count is 150,000–350,000 per mm3.
The two most common ways to collect blood are phlebotomy and capillary puncture. Phlebotomy or venipuncture requires insertion of a needle into a superficial vein to withdraw blood. Phlebotomy is performed primarily for diagnosis and monitoring a patient’s condition. Capillary puncture involves a puncture (prick) of the skin with a lancet, which causes the body to release a small amount of blood.
If a large blood specimen is needed, venipuncture is the preferred method. A vein is punctured with a needle and blood is collected utilizing the evacuated tube method, the syringe method, and/or the butterfly method. The evacuated tube method allows for the collection of several samples from one venipuncture site using interchangeable vacuum collection tubes. Collection tubes are calibrated by evacuation to collect the exact amount of blood required for a specific test. Tube volumes range from 2 to 15 mL. Some tubes are prepared with additives to correctly process blood samples for testing. There is no need to transfer materials from a collection syringe or a sample tube.
The evacuated tube system includes an evacuated closed glass or plastic tube containing a vacuum, a special double-pointed needle, and a plastic needle holder/adapter. The vacuum tubes contain several different additives such as clot activators, anticoagulants, and thixotropic gels.
The size and content of the tubes depends upon the test to be performed. With the exception of the red-stoppered tubes, all tubes contain an additive. Clot activators promote clotting of blood by providing a surface for platelet activation. Thrombin encourages clotting and is used in tubes drawn for stat chemistry testing. Anticoagulants prevent blood from clotting and can be used to collect whole blood and plasma.
Despite the method of blood collection, the samples must be mixed with the appropriate additive in the correct collection tubes before they are transported to the lab for testing. Each tube stopper has a different color, each color identifying the type of additives they contain. Tubes should be filled in a specific order to preserve the integrity of the blood sample. The following table outlines the National Committee for Clinical Laboratory Standards recommended order of draw.
| Order | Tube stopper color | Additive | Tests |
| 1 | Yellow | Sodium polyanetholesulfonate | Blood cultures |
| 2 | Light blue* | Sodium citrate additive | Coagulation studies |
| 3 | Red | No additive | Chemistry, AIDS antibody, viral studies, serology,
blood typing |
| 4 | Red/gray (“tiger”) |
Silicone serum separator | Serum testing |
| 5 | Green | Sodium heparin (anticoagulant) | Electrolytes, arterial blood gases |
| 6 | Lavender/purple/pink |
EDTA** |
Hematology, blood bank crossmatching |
| 7 | Gray |
Potassium oxalate or sodium fluoride | Blood glucose |
| * If a light blue–stoppered tube is the first or only tube to be drawn, a 5-mL, red-stoppered tube should be drawn first and discarded to eliminate contamination. This is because the needle may pick up thromboplastin (clotting factor) from the patient’s body when it penetrates the skin. Thromboplastin can then enter the blood specimen, affecting test results. | |||
| **EDTA = ethylenediaminetetraacetic acid | |||
Capillary blood specimen is used when a small amount of blood is required. Small blood samples can be collected for testing hemoglobin, hematocrit, and glucose. The skin puncture technique can also be used for adults if there are no accessible veins suitable for venipuncture. Infants and children typically have low blood volume and removing large quantities may result in anemia.
Puncture sites will vary depending on the age of the patient. For adults and children, the typical site is the distal phalynx of the middle (3) or ring finger (4) on the nondominant hand. Capillary puncture on infants (birth to one year) is performed on the outer edge of the plantar surface of the heel or the big toe. Capillary puncture is not performed on the finger because the amount of tissue between skin surface and bone is too small and may injure the bone. Once a child begins to walk, the fingertip is the preferred site.
The skin puncture technique is performed using a disposable semiautomatic lancet. The disposable semiautomatic lancet is a spring-loaded plastic holder with a metal blade. This device is available in various sized blades, which are used to control the pierce depth. Blades are approximately 1.0-mm wide and produce a small cut that results in an ample blood flow. The blade itself is concealed within a plastic case. The typical puncture depth should not penetrate deeper than 3.1 mm on an adult and 2.4 mm on infants and children.
Blood specimens can be collected directly onto a reagent strip (glucose testing), into capillary tubes, and/or microcollection tubes. Capillary tubes are disposable glass or plastic tubes that can hold 5 to 75 microliters of blood. These tubes are most often used for hematocrit determination, but have many other uses, such as cholesterol testing. Microcollection tubes are small plastic tubes having a removable collector tip. The tip is designed to collect capillary blood from a skin puncture.
The patient should be seated comfortably. The patient’s arm is extended with the palm surface of the hand facing upward, exposing the lateral part of the tip of the third or fourth finger on the nondominant hand. Gently massage the finger or place in warm water to increase blood flow. The first drop of blood is wiped away with gauze, as the sample is diluted with alcohol and is unsuitable for testing. The medical assistant should allow a large drop of blood to form. The blood droplet should then be collected and placed in the appropriate device.
Several blood tests are routinely ordered as part of a complete examination to determine a patient’s health. Medical assistants must understand the chemical properties, the purpose, and the normal ranges for each of those tests. Hematologic tests include blood cell counts, morphologic studies, coagulation tests, and erythrocyte sedimentation rates. These tests can be performed on venous or capillary whole blood samples. A complete blood cell count (CBC) includes the following items: white blood cell count (WBC), red blood cell count (RBC), platelet count, hemoglobin (Hgb), hematocrit (Hct), differential white blood cells count (Diff), and red blood cell indices.
Morphological studies assess the shape or form of cells. A blood smear is examined and the cells are studied for appearance and shape. Abnormal size, shape, content, and color are noted. A morphologic study is often performed on the same blood smear slide after the differential count and platelet estimate.
Erythrocyte sedimentation rate (ESR) measures the rate (time) it takes red blood cells to settle to the bottom of a blood sample. A freshly collected sample of anticoagulated blood is transferred to a calibrated tube and placed in a sedimentation rack on a counter free of vibrations and away from sunlight. The tube is examined exactly one hour later to determine the distance the red blood cells have fallen. Tests results are recorded as millimeters per hour (mm/hr). This test can indicate inflammation within the body.
Hemoglobin (Hgb or Hb) measures the oxygen-carrying capacity of the blood. The normal range for an adult woman is 12–16 g/dL and for an adult man is 14–18 g/dL. A decreased level occurs with anemia (especially iron deficiency), hyperthyroidism, and certain systemic diseases such as leukemia. Increased levels of hemoglobin are present with polycythemia, chronic obstructive pulmonary disease (COPD), and congestive heart failure.
Hematocrit (Hct) means to separate blood. The solid or cellular elements are separated from the plasma by centrifuging an anticoagulated blood specimen. Red blood cells settle at the bottom of the tube. The top layer contains plasma. Hematocrit measures the volume of packed red blood cells in whole blood. The normal range for an adult woman is 37–47 percent and for an adult male is 40–54 percent. A low level may indicate anemia and an elevated level may indicate polycythemia. Calculating the hematocrit requires dividing the amount of packed red blood cells by the amount of the whole blood volume and multiplying by 100.
White blood cell differential count is used to identity the five types of white blood cells within a sample. A blood cell counter is used to determine the number of neutrophils, lymphocytes monocytes, eosinophils, and basophils. Neutrophils are granulocytes that have a purple, multilobed nucleus. Neutrophils are seen with acute infection. Lymphocytes are agranulocytes with a round or slightly indented nucleus that almost completely fills the cell and stains a deep purplish blue. Lymphocytes are seen with viral disease, infectious mono, mumps, chicken pox, rubella, and viral hepatitis. Monocytes are agranulocytes that contain a large nucleus that is kidney or horseshoe-shaped. Monocytes are the largest phagocytic white blood cell. Eosinophils are granulocytes with a segmented nucleus. These cells are seen with allergic conditions and parasitic infestation. Basophils are also granulocytes that contain an s-shaped nucleus.
Blood chemistry testing involves the quantitative measurement of chemical substances present in the blood. These chemicals are dissolved in the liquid portion of the blood; therefore, most blood chemistry tests require a serum specimen for analysis. Automated blood chemistry analyzers consist of a reflectance photometer that quantitatively measures the amount of chemical substances or analytes in the blood.
Several blood chemistry tests are routinely performed in the physician’s office laboratory. Glucose monitoring is usually performed on a capillary blood specimen and the level is determined by comparing the color on a reagent strip to a standard or by using an automated device. Hemoglobin A1c is another test used to monitor diabetic patients. The test measures the amount of glycosylated hemoglobin in the blood. As blood glucose levels rise, the glucose molecules bind with hemoglobin to form hemoglobin A1c (HgBA1c). This test can be performed without fasting. HgBA1c gives information over a period of two to three months. The ideal range for HgBA1c is less than 7 percent for a diabetic patient. A level below 5.7 percent would be expected of a nondiabetic patient.
Fasting blood sugar (FBS) involves collecting a blood sample and measuring the amount of glucose present. The patient should not have anything to eat or drink, except water, for 12 hours preceding the test. Certain medications interfere with FBS and should not be taken for the three days leading up to collection of the FBS sample. These medications include salicylates (aspirin), diuretics, and steroids. Normal range is 70–100 mg/dL. An FBS of 100–125 mg/dL is considered prediabetes; if FBS is 126 mg/dL or greater on two separate tests, the diagnosis of diabetes is likely.
The two-hour postprandial blood sugar test is used to screen for the presence of diabetes mellitus and to monitor the effects of insulin dosage in diagnosed diabetics. The patient is required to fast. A blood specimen is collected two hours after consumption of a 100-gram test-load glucose solution. In a nondiabetic patient, the glucose level returns to the fasting level within one and a half to two hours from the time of glucose consumption. The glucose level in the diabetic patient does not return to the fasting level. A postprandial glucose level of 140 mg/dL or higher is suggestive of diabetes mellitus.
Glucose tolerance test (GTT) provides more detailed information about the ability of the body to metabolize glucose by assessing the insulin response to a glucose load. The patient is required to consume a high carbohydrate diet for three days, consisting of 150 grams of carbohydrates per day. The patient must be in the fasting state when the test begins. After the FBS has been performed, the patient is instructed to drink a measured amount of a glucose solution (1.75 grams of glucose per kilogram of body weight, or the standard adult dose of 100 grams). Thereafter, at regular intervals (30, 60, 120, and 180 minutes), blood and urine samples are taken. The patient should not consume food or fluid (except water) during the test to assist in producing urine samples. Smoking is not permitted because it acts as a stimulant that increases the blood glucose level. The patient should remain at the site because activity affects the test results by utilizing glucose. In a nondiabetic patient, the blood glucose level rises to a range between 160 and 180 mg/dL approximately 30 to 60 minutes after the glucose solution is consumed, but should return to normal levels within two hours.
Cholesterol is a white, waxy, fat-like substance (lipid) that is essential for normal functioning of the body. Cholesterol levels are a combined measurement of the amount of low-density lipid cholesterol (“bad cholesterol”) and high-density lipid cholesterol (“good cholesterol”) in the blood. Cholesterol levels vary by age, but for the average adult test results are interpreted as follows: under 200 mg/dL is desirable, levels 200–239 mg/dL are considered borderline high, and 240 mg/dL and above are considered high. Patients are not required to fast for total cholesterol and HDL testing; however, for a complete lipid profile, the patient must fast. Total cholesterol and high-density lipid cholesterol determinations are not affected significantly by food consumption. If the patient is getting a lipid profile done, they must be in a fasting state. The lipid profile includes total cholesterol, triglycerides, high density lipoprotein (HDL), and low density lipoprotein (LDL). Triglycerides are affected by food intake; a level higher than 250 mg/dL is considered dangerous.
The two most common blood tests for kidney function are the blood urea nitrogen test (BUN) and the creatinine test. These two substances are produced by cellular metabolism and the kidney should clear them out of the blood via urine. High levels of these substances in the blood therefore suggest that kidney dysfunction.
Another group of tests indirectly assesses the health of liver cells by measuring enzymes levels and substances produced by the liver. Liver function tests include alanine aminotransferase (ALT), aspartate aminotransferase (AST), gamma-glutamyltranspeptidase (GGT), bilirubin levels, albumin levels, and prothrombin time (PT).
ALT is an enzyme produced in hepatocytes, the major cell type in the liver. AST is an enzyme found in muscle. In many cases of liver inflammation, the ALT and AST activities are elevated roughly in a 1:1 ratio. GGT is an enzyme produced in the bile ducts that may be elevated in the serum of patients with bile duct diseases. Bilirubin is removed from the blood by the liver. Bilirubin concentrations are elevated in the blood either by increased production, decreased uptake by the liver, decreased secretion from the liver, or blockage of the bile ducts. Albumin is synthesized by the liver and secreted into the blood. Low serum albumin concentrations indicate poor liver function. Prothrombin time (PT) is necessary for blood clotting and is made in the liver. In chronic liver diseases, the prothrombin time is usually not elevated until cirrhosis is present.
Serology is defined as the scientific study of serum of the blood. Mononucleosis is caused by the Epstein-Barr virus. Individuals with mononucleosis produce an antibody called heterophile antibody, usually by the 6th to 10th day of the illness. Rapid mono tests are able to detect the presence of this antibody within five minutes.
ABO and Rh blood typing is performed to determine an individual’s ABO and Rh blood type. Blood typing helps to prevent transfusion and transplant reactions, helps identify problems such as hemolytic disease of newborns, and can assist in determining parentage. The Rh Antibody Titer detects the amount of circulating Rh antibodies against red blood cells. These antibodies can occur in a pregnant woman who is Rh– and is carrying an Rh+ fetus.