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Surgical Site Infection: Pathogenesis and Prevention

  • Authors: Author: Donald E. Fry, MD
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Target Audience and Goal Statement

This activity is intended for surgeons and nurses.

The goal of this activity is to provide current strategies for the prevention of surgical site infections.

Upon completion of this self-study activity, participants will be able to:

  1. Discuss the pathogenesis of surgical site infections.
  2. Explain the determinants of surgical site infections.
  3. Describe the prevention of surgical site infections.

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    For Nurses

  • 1.2 contact hours of continuing education for RNs, LPNs, LVNs, and NPs. This activity is cosponsored with Medical Education Collaborative, Inc. (MEC) and Medscape. MEC is accredited as a provider of continuing nursing education by the American Nurses Credentialing Center's Commission on Accreditation.
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    California Board of Registered Nursing, Provider Number CEP 12990, for 1.2 contact hours.

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Surgical Site Infection: Pathogenesis and Prevention


Classification and Risk of SSI

Different surgical sites may contribute to the risk of developing clinical infection. For example, cosmetic operations of the head and neck in otherwise healthy patients pose a much lower risk of SSI than colon resection for cancer in an elderly patient with chronic obstructive lung disease and obesity. Elective procedures have lower SSI rates than do emergency procedures. Stratification of various operations into groups that have similar risks for infection is important so that preventive strategies can be appropriately evaluated among similar patients, and so that quality monitors can be implemented to identify when infection rates are at variance from accepted trends and norms within an institution. An assessment of gross SSI rates without stratification is of only limited value, since overall rates are likely to be a reflection of patient risk rather than quality of performance.

The traditional wound infection classification system was developed in the wake of the ultraviolet light study of 1964.[9] This classification system was primarily designed to provide a clinical estimate of the inoculum of bacteria likely to be encountered during the procedure and does not address the other determinants of infection defined above. Four separate classes of procedures were identified, each with a unique infection rate.

Clean Wounds

The wound is judged to be clean when the operative procedure does not enter into a normally colonized viscus or lumen of the body. Elective inguinal hernia repair is an example of a clean operative procedure. SSI risk is minimal and originates from contaminants of the OR environment or from the surgical team, or most commonly from skin colonists. The most common pathogen is Staphylococcus aureus. SSI rates in this class of procedures should be 2% or less, depending upon other clinical variables.

Clean-Contaminated Wounds

A clean-contaminated surgical site is seen when the operative procedure enters into a colonized viscus or cavity of the body, but under elective and controlled circumstances. The most common contaminants are endogenous bacteria from within the patient. For example, sigmoid colectomy wounds generally contain E coli and Bacteroides fragilis as microbial contaminants. Elective intestinal resection, pulmonary resection, gynecologic procedures, and head-neck cancer operations that involve the oropharynx are examples of clean-contaminated procedures. Infection rates for these procedures are in the range of 4% to 10% and can be optimized with specific preventive strategies.

Contaminated Wounds

Contaminated procedures occur when gross contamination is present at the surgical site in the absence of obvious infection. Laparotomy for penetrating injury with intestinal spillage and elective intestinal procedures with gross contamination of the surgical site are examples of contaminated procedures. As with clean-contaminated procedures, the contaminants are the bacteria that are introduced by gross soilage of the surgical field. Infection rates will be greater than 10% for this classification of wound, even with preventive antibiotics and other strategies.

Dirty Wounds

Surgical procedures performed when active infection is already present are considered dirty wounds. Abdominal exploration for acute bacterial peritonitis and intra-abdominal abscess are examples of this class of surgical site. Pathogens to be expected are the pathogens of the active infection that is encountered. Unusual pathogens are often encountered in dirty wounds, especially if the infection has occurred in a hospital or nursing home setting, or in patients receiving prior antibiotic therapy.

To further allow comparison of infection rates between institutions and analyses of SSI rates within a given institution over time, the US Centers for Disease Control and Prevention (CDC) developed the NNIS Risk Index system, by which member hospitals report cumulative wound infection data. A risk index has been developed to include the traditional wound classification system defined above and additional variables.[10] This simplified risk index has a range from 0 to 3 points. A point is added to the patient's risk index for each of the following 3 variables:

  • 1 point - the patient has an operation that is classified as either contaminated or dirty.

  • 1 point - the patient has an American Society of Anesthesiologists (ASA) preoperative assessment score of 3, 4, or 5 (Table 1)

  • 1 point - the duration of the operation exceeds the 75th percentile where a standard T point (75% percentile) was determined from the NNIS database (Table 2); the T point is defined as the length of time in hours that represents the 75th percentile of procedures reported in the NNIS survey[10]

Table 1. Physical Status Classification for Surgical Patients

Class I A patient in normal health
Class II A patient with mild systemic disease resulting in no functional limitations
Class III A patient with severe systemic disease that limits activity, but is not incapacitating
Class IV A patient with severe systemic disease that is a constant threat to life
Class V A moribund patient not likely to survive 24 hours

Table 2. The T Point for Common Surgical Procedures

Operation T Point (hrs)
Coronary artery bypass graft 5
Bile duct, liver, or pancreatic surgery 4
Craniotomy 4
Head and neck surgery 4
Colonic surgery 3
Joint prosthesis surgery 3
Vascular surgery 3
Abdominal or vaginal hysterectomy 2
Ventricular shunt 2
Herniorrhaphy 2
Appendectomy 1
Limb amputation 1
Cesarean section 1

The NNIS Risk Index has the advantage of using the ASA preoperative assessment score as an estimate of the patient's overall health at the time of the operation. The duration of the procedure becomes a surrogate marker for procedures of unusual complexity. The NNIS Risk Index has become a standard format for presenting SSI data by many institutions and is largely replacing the older wound classification system, while still using the older system as part of its methodology (Table 3).

Source: National Nosocomial Infections Surveillance (NNIS) System Report, data summary from January 1992 to June 2002, issued August 2002.
Centers for Disease Control, Atlanta, Georgia.

Table 3. SSI Risk Stratification By Type of Surgery and T Point

Risk Category
Type of Operation T Point (hrs) 0 1 2 3
Colon surgery 3 3.2 8.5 16.0 22.0
Vascular surgery 3 1.6 2.1 6.1 14.8
Cholecystectomy 2 1.4 2.0 7.1 11.5
Organ transplant 7 0.0 4.4 6.7 18.0

Source: National Nosocomial Infections Surveillance (NNIS) System Report, data summary from January 1992 to June 2002, issued August 2002.
Centers for Disease Control, Atlanta, Georgia.