Surgical Site Infections

Laura H. Rosenberger, MD, MS and Robert G. Sawyer, MD

Overview

Surgical site infections (SSIs) are a common surgical complication that affects between 2% and 5% of the 30 to 40 million operations that occur in the United States per year. SSIs are the most common nosocomial infection among surgical patients and are consistently the second most common healthcare-associated infection overall. Mortality rates after SSI are markedly higher when compared with patients without an SSI, as are the patient's length of stay (mean, 7 days), hospital readmission rates, and direct patient costs ($500 to $3000 per infection).

Definition

Controversy and ambiguity continue to exist in the definition of an SSI. Experts may disagree about the appearance of an incision in regards to SSI, particularly in the presence of surrounding cellulitis or wound drainage. In 1970, the Centers for Disease Control and Prevention (CDC) established the National Nosocomial Infections Surveillance (NNIS) system (now part of the National Healthcare Safety Network [NHSN]) in part to create criteria defining SSI and to standardize the definition for accurate surveillance and reporting. Currently, it is the national standard by which medical personnel and researchers consistently define SSI.

By definition, SSIs occur within the first 30 days after the operation if no implant is placed or within 1 year if an implant remains. The classification scheme defines the following two categories of SSI based on location: incisional SSI (subdivided into superficial and deep) and organ/space SSI (Figure 1). Specific criteria for defining each of the categories of SSI infection can be found in the CDC's Guidelines for Prevention of Surgical Site Infection.

FIGURE 1 Cross section of abdominal wall depicting Centers for Disease Control classifications of surgical site infection (SSI). (From Mangram AJ, Horan TC, Pearson ML, et al: Guideline for prevention of surgical site infection, Infect Control Hosp Epidemiol 20(4):247-278, 1999.)

Risk Factors

Development of an SSI requires microorganism contamination at the surgical site. During an operative procedure in which skin is incised, endogenous skin flora, the most common source of pathogens, are introduced into the exposed tissue. Additional sources of bacteria include patient colonization, mucous membrane or hollow viscous pathogens encountered during the operation, surgical personnel, operative instruments, and the operating room environment.

An accepted surrogate for bacterial contamination at the surgical site is the wound classification. The risk of SSI increases with the degree of contamination and higher wound classifications. Wounds are classically defined as clean, clean-contaminated, contaminated, or dirty/infected, as seen in Table 1. Numerous additional characteristics contribute to the risk of SSI development, including patient factors, environmental factors, and treatment factors, as seen in Table 2.

TABLE 1:

Surgical wound classifications

Surgical wound classification	Description
Class I/clean	An uninfected operative wound in which no inflammation is encountered and the respiratory, alimentary, genital, or uninfected urinary tract is not entered. In addition, clean wounds are closed primarily and, if necessary, drained with closed drainage.
Class II/clean-contaminated	An operative wound in which the respiratory, alimentary, genital, or urinary tracts are entered under controlled conditions and without unusual contamination. Specifically, operations that involve the biliary tract, appendix, vagina, and oropharynx are included in this category, provided no evidence of infection or major break in technique is encountered.
Class III/contaminated	Open, fresh, accidental wounds. In addition, operations with major breaks in sterile technique (e.g., open cardiac massage) or gross spillage from the gastrointestinal tract and incisions in which acute, nonpurulent inflammation is encountered are included in this category.
Class IV/dirty, infected	Old traumatic wounds with retained devitalized tissue and those that involve existing clinical infection or perforated viscera. This definition suggests that the organisms that cause the postoperative infection were present in the operative field before the operation.

Modified from Mangram AJ, Horan TC, Pearson ML, et al: Guideline for prevention of surgical site infection, Infect Control Hosp Epidemiol 20(4):247-278, 1999.

TABLE 2:

Patient and operative characteristics that may influence the risk of surgical site infection development

Patient factors	Environmental factors	Treatment factors
Ascites	Contaminated medications	Emergency procedure
Chronic inflammation	Inadequate disinfection/sterilization	Failure to obliterate dead space
Coexistent remote infection	Inadequate skin antisepsis	Hypothermia
Colonization with microorganisms	Inadequate ventilation	Inadequate antibiotic prophylaxis
Corticosteroid therapy		Intraoperative blood transfusion
Diabetes		Oxygenation
Extended preoperative admission		Poor hemostasis
Hypocholesterolemia		Prolonged operative time
Hypoxemia		Surgical drains
Malnutrition		Tissue trauma
Obesity
Peripheral vascular disease
Perioperative anemia
Preoperative shaving
Prior site irradiation
Recent operation
Skin disease in the area of infection (e.g., psoriasis)

Modified from the National Nosocomial Infections Surveillance (NNIS) system report: data summary from January 1992-June 2001, issued August 2001, Am J Infect Control 29:404-421, 2001; and from Mangram AJ, Horan TC, Pearson ML, et al: Guideline for prevention of surgical site infection, Infect Control Hosp Epidemiol 20(4):247-278, 1999.

The NHSN developed a risk index by which the risk of an SSI can be predicted based on three major criteria: wound classification, American Society of Anesthesiologists (ASA) score, and duration of the operation. The SSI risk category is based on the number of factors present at the time of operation, including a wound class of 3 (contaminated) or 4 (dirty), an ASA class of 3 or greater, and an operation lasting longer than the 75th percentile of the duration of the specific operation. Each independent factor is given a single point if present, which determines the NHSN risk index category (0 to 3). As seen in Table 3, the risk of SSI increases with progressive wound classifications and with progressive NHSN score.

TABLE 3:

National Healthcare Safety Network (NHSN) risk index scoring

NA, Not applicable; NNIS, National Nosocomial Infections Surveillance system.

^*NHSN risk index is based on three independent variables: American Society of Anesthesiologists (ASA) score > 2, contaminated or dirty/infected wound class, and length of operation above the 75th percentile for the operation being performed.

Modified from data presented in National Nosocomial Infections Surveillance (NNIS) system report: data summary from January 1992-June 2001, Am J Infect Control 29(6):404-421, 2001.

The laparoscopic approach to an operation has been associated with a decreased risk of SSI as a result of small wound size, limited use of cautery, and decreased inflammatory response to tissue injury. This decrease in SSI risk has led to a modification of the risk classification. For laparoscopic biliary, gastric, or colon operations, a –1 is applied, essentially subtracting one risk factor when the operation is completed via a laparoscope.

Prevention

Patient Preparation

Prevention of SSIs should start as early as the preoperative clinic visit. Patient factors that may be modified before surgery should be corrected when possible. This includes clearing of remote site infections; initiation of an active smoking cessation program; optimization of nutritional status, including potential supplemental enteric feeding; weight loss for obese patients; whole body showering with a chlorhexidine antimicrobial solution; and prohibition of shaving just before the operation. Although not all of these interventions have proven benefits, they are simple inexpensive interventions that involve patients in their own medical care, an invaluable contribution to SSI prevention.

Immediate Preoperative Prevention

In response to inconsistent compliance with proven infection prevention measures, the Centers for Medicare and Medicaid Services (CMS) collaborated with the CDC on the Surgical Infection Prevention (SIP) project in 2002. This project developed quality improvement measures including the timeliness, selection, and duration of prophylactic antibiotics. In 2006, the Surgical Care Improvement Project (SCIP) developed out of the SIP and its process measures. The SCIP initiatives were published in the Specifications Manual for National Inpatient Quality Measures (Specifications Manual) and provided a standard form of quality measures to document and track standards of care (available online at www.qualitynet.org). The manual outlined seven infection-related process measures applicable to the perioperative period (Table 4). The initiatives that are outlined in SCIP infection measures (INF) 1 to 4, 6, and 10 highlight important process measures to prevent SSIs.

TABLE 4:

Surgical Care Improvement Project (SCIP) performance measures applicable to the perioperative period

SCIP Infection (INF) measure designation	Performance measure title
INF 1	Prophylactic antibiotic received within 1 hour before surgical incision
INF 2	Prophylactic antibiotic selection for surgical patients
INF 3	Prophylactic antibiotics discontinued within 24 hours after surgery end time
INF 4	Cardiac surgery patients with controlled 6:00 am postoperative blood glucose
INF 6	Surgery patients with appropriate hair removal
INF 9	Urinary catheter removed on postoperative day 1 or postoperative day 2 (with day of surgery being day 0)
INF 10	Surgery patients with perioperative temperature management

Modified from Rosenberger LH, Politano AD, Sawyer RG: The surgical care improvement project and prevention of post-operative infection, including surgical site infection, Surg Infect 12(3):163-168, 2011.

Prophylactic Antibiotics (SCIP INF 1 to 3)

Numerous studies have shown that provision of antimicrobial prophylaxis within 1 hour before the surgical incision allows blood and tissue concentrations adequate to prevent SSIs. A number of studies revealed enhanced compliance with antibiotic timing significantly reduced postoperative infectious complications, including SSIs. One notable prospective multicenter study attempted to define the optimal timing for antimicrobial prophylaxis and found 0 to 30 minutes to be associated with the fewest SSIs (1.6%). They found the infection risk increased with an extended interval between administration of antimicrobials and incision and with antimicrobial prophylaxis given after incision (Table 5).

TABLE 5:

Association between timing and surgical site infections for antimicrobial prophylaxis, cephalosporins, or antimicrobials designated to be given within 60 minutes of incision

Timing of antimicrobial prophylaxis	Infections/no. of operations	Infection risk
>120 min before incision	4/96	4.7%
61-120 min before incision	12/489	2.4%
31-60 min before incision	38/1558	2.4%
0-30 min before incision	22/1339	1.6%
1-30 min after incision	4/100	4.0%
>30 min after incision	5/74	6.8%

Modified from Steinberg JP, Braun BI, Hellinger WC, et al: Timing of antimicrobial prophylaxis and the risk of surgical site infections, Ann Surg 250(1):10-16, 2009.

Prophylactic antimicrobials should provide coverage for the pathogens most likely to be encountered during the operation, have pharmacokinetics that ensure adequate serum and tissue concentrations, have demonstrated safety, and be cost effective. The class of antibiotics most commonly used for perioperative prophylaxis is the cephalosporins. They are effective against gram-positive and gram-negative pathogens commonly encountered in general surgery and meet other criteria for appropriate prophylactic antimicrobials. Cefazolin is generally the agent of choice for clean and many clean-contaminated cases; a second-generation cephalosporin such as cefoxitin is commonly selected for colon operations for additional anaerobic coverage. For specific classes of operations such as cardiac or orthopedic operations, the antibiotic selection becomes more complex. The SCIP released a pocket card (October 2010) to guide the selection of appropriate antimicrobials based on operative location, class, and potential allergies (Table 6). The authors recommend the surgeon consult a pathogen reference table to determine prophylaxis based on operative type and most likely pathogens encountered (Mangram and colleagues).

TABLE 6:

Surgical Care Improvement Project (SCIP) pocket card: prophylactic antimicrobial regimen selection for surgery

This table does not necessarily reflect Centers of Medicare and Medicaid Services policy.

^*Vancomycin is acceptable with a physician, nurse practitioner, physician assistant, or pharmacist documented justification for its use.

^†For cardiac, orthopedic, and vascular surgery, if the patient is allergic to β-lactam antibiotics, vancomycin and clindamycin are acceptable.

^‡A single dose of ertapenem is recommended for colon procedures.

Modified from the Oklahoma Foundation of Medical Quality, the Quality Improvement Organization Support Center for Patient Safety, under contract with the Centers of Medicare & Medicaid Services (CMS), an agency of the U.S. Department of Health and Human Services: Surgical care improvement project pocket card, October 2010.

The discontinuation of prophylactic antimicrobials is also important in terms of infection control and outcomes regarding SSIs. Significant evidence shows no additional benefit to lengthening the course of prophylactic antimicrobials beyond 24 hours. This is consistent with the rationale that perioperative antibiotics are not meant to sterilize the tissue but rather to reduce the bacterial burden to an inoculum that may be controlled by the patient's defenses. Extending the length of prophylaxis increases the risk of drug resistance and secondary infections, such as Clostridium difficile–related disease; therefore, the authors recommend the discontinuation of prophylactic antibiotics after 24 hours.

Treatment

All clearly infected wounds should be opened, irrigated, débrided, and treated with basic wound care. Most superficial and deep incisional SSIs can be managed with these techniques. The wound should be opened sufficiently for adequate visualization of the underlying tissue to ensure no additional processes are occurring, such as fascial dehiscence, drainage from an organ/space SSI, or enteric fistula.

Infected incisional wounds should be irrigated with an isotonic solution such as a saline solution to remove loose dead tissue and exudate, and mechanical débridement may be indicated to remove devitalized tissue. Wounds should be packed in a “wet to dry” fashion, with moistened gauze against the wound bed covered by layers of dry gauze. Daily dressing changes assist in débridement as the wet gauze removes residual necrotic tissue and exudate. Once the wound has stabilized and most of the devitalized tissue has been removed, consideration can be given to placement of a negative-pressure wound therapy device (“wound vac”). Although the overall outcome may not change, placement of such a device generally speeds closure and may be particularly helpful in larger wounds and those in which dressing changes are difficult, such as with the distal extremities.

The need for antimicrobial therapy is determined by magnitude of the infection, evidence of systemic involvement, presence of prosthetics, and status of the patient, including comorbidities. Uncomplicated SSIs that have been managed with incision and drainage can usually be managed without antibiotics. Topical antibiotics and additional agents such as antiseptics (hydrogen peroxide, povidone-iodine) have an unclear role in the management of infected wounds. Traditionally, many antiseptics have been applied to wounds, including iodine-containing solutions and sodium hypochlorite (Dakin's solution). These compounds can inhibit fibroblast growth in vitro and are only used for a relatively short period of time. Topical antibiotics, as opposed to antiseptics, probably have no role in the management of infected wounds. In resource-poor areas, both honey and granulated sugar have been used to treat open wounds, and their effectiveness may be based on their hypertonicity. Enzymatic débridement of wounds may be beneficial where aggressive wide surgical débridement is impossible.

For moderate to severe SSIs, including those associated with systemic toxicity, significant cellulitis (>2 cm beyond the incision), purulent drainage, fascial dehiscence, and deep drainage, antibiotics should be administered empirically. Initial selection should cover gram-positive organisms and likely infecting organisms based on colonization status, operative location, and recent infections. For wounds associated with operations on the gastrointestinal or genitourinary tracts, gram-negative coverage should also be provided. Source control is imperative for organ/space SSIs, including intraabdominal abscess, and requires either percutaneous or operative drainage.

Culture of wounds is at the discretion of the surgeon. Straightforward uncomplicated incisional SSIs without significant cellulitis that do not require antimicrobial therapy do not need to be cultured. On the other hand, all organ/space SSIs should be cultured because the role of antibiotics is greater in this instance. For other cases, cultures should only be sent when antibiotics are planned, the culture results will be used to guide antibiotic choice, and the patient is at high risk of having resistant pathogens. Patient characteristics that suggest culture might be indicated include recent receipt of therapeutic antibiotics, immunosuppression, care in an intensive care unit, and an unusually aggressive infection. Ideally cultures should be sterile aspirations or tissue, but swabs are useful if cautious interpretation of the results is used.

Summary

SSIs are a common surgical complication and represent the most common nosocomial infection among surgical patients. These infections cause significant morbidity and increase costs, making them the focus of discussion and regulation at various levels. A number of process measures have been identified to reduce the risk of SSIs, and evidence-based standards should be incorporated into all surgeons' practices.