Surgical site infection (SSI) is one form of post-operative morbidity. It is a major risk in orthopaedic surgery. SSI causes pain and can lead to wound
dehiscence and generalized sepsis. Further surgery and admission to intensive care may be necessary. A patient with a SSI spends twice the average length of time in hospital31. SSI is therefore not only distressing for the patient; it is also an economic burden for the health care provider.
Superficial wound infection can spread to deeper tissues including bone. A deep infection diagnosed within the first six weeks of primary joint replacement (hip or knee) is treated with repeated joint washouts, replacement of
polyethylene components and intravenous antibiotics. These interventions may be sufficient to eradicate the infection. If a deep infection is diagnosed after the sixth post-operative week, revision joint replacement surgery is normally
required (one-stage or two-stage) together with a prolonged course of intravenous antibiotics32. If the infection cannot be eradicated, life-long antibiotics to suppress the infection, joint arthrodesis (fusion) or even limb amputation may be required.
Wound surveillance in Orthopaedic Surgery became mandatory in the NHS in England in 2004. Reported SSI rates depend on the method used for
diagnosis, case mix, the thoroughness of surveillance and documentation and the length of patient follow-up. Patient follow-up is essential in any wound
33 surveillance program since half of SSIs present after hospital discharge33. Therefore SSI rates cannot be defined as a ‘short-term’ outcome measure. Long-term follow-up of patients must be established to ensure true rates are reported.
There is a misconception that SSIs are simple to define and diagnose. Several definitions of SSI have been proposed and diagnosis varies between surgeons. Diagnosis cannot rely solely on microbiology results since this would delay the initiation of appropriate treatment. There would also be false positive results from contaminants and false negative results when organisms fail to grow in the culture medium.
SSIs were traditionally diagnosed using the hallmarks of pain (dolor), redness (rubor), heat (calor), swelling (tumor) and impairment of function. As surgeons became increasingly accountable for their practice, more reliable and
reproducible methods of diagnosing SSI became necessary. Three SSI definitions in use today are the American Centres for Disease Control (CDC) definition, the English Nosocomial Infection National Surveillance Scheme (NINSS) definition and the English ASEPSIS definition. ASEPSIS is an acronym for Additional treatment, Serous discharge, Erythema, Purulent exudate,
Separation of deep tissues, Isolation of bacteria and Stay as inpatient prolonged over fourteen days.
34 The CDC definition34 is used worldwide to classify wound infections. It includes any wound infection within 30 days of surgery or one year if an implant is
present. The CDC definition divides SSIs into incisional and organ/space infections. Incisional SSIs are further divided into superficial and deep
infections (see table 4). Although widely used, the CDC definition is weak since three out of the four diagnostic criteria are subjective. On psychometric
evaluation CDC has been shown to be unreliable35.
Table 4. CDC definition of surgical site infection Superficial infection
(involving skin and superficial tissues)
Deep infection
(involving the fascial and muscle layers) Time period Occurs within 30 days of
surgery
Occurs within 30 days of surgery or within 1 year if implant present
Site Involves only the skin and superficial tissue
Related to the surgical site and involves deep tissues
Further criteria
Must fulfil one of the following: Purulent discharge from superficial incision Organisms isolated from incision Pain, tenderness, swelling, redness or heat around the incision AND the incision deliberately opened by a surgeon (unless cultures are negative)
Diagnosis by a surgeon or physician
Must fulfil one of the following: Purulent discharge from
deep incision Spontaneous
dehiscence or deliberate opening of a deep
incision, following fever or pain or tenderness around the wound (unless cultures are negative)
Abscess involving a deep incision
Diagnosis by a surgeon or physician
35 The UK NINSS definition of SSI is based on CDC with two significant
modifications. Firstly, pus cells must be present for a wound culture to be classified as positive. Secondly, a surgeon’s diagnosis of infection is excluded as a sufficient criterion to diagnose SSI. These changes were implemented to improve the objectivity of CDC but reproducibility of NINSS remains low36.
ASEPSIS is a quantitative wound scoring method developed in 198637. It provides a numerical score that indicates the severity of wound infection. The score is calculated using objective criteria based on the wound’s physical appearance (e.g. erythema and serous exudate) and the clinical consequences of infection (e.g. prolonged hospital stay and readmission) (tables 5 and 6). A score of over 10 indicates an increasing probability and severity of infection (table 7). The original ASEPSIS scoring method was psychometrically tested and found to be objective and repeatable38. The most recent revised version has not undergone the same evaluation.
36
Table 5. Points scale used to calculate ASEPSIS score
Criterion Points
Additional treatment Antibiotics 10 Drainage of pus under local
anaesthetic
5
Debridement of wound under general anaesthetic
10
Serous discharge 0-5
Erythema 0-5
Purulent exudates 0-10
Separation of deep tissues 0-10
Isolation of bacteria 10
Stay in hospital over 14 days 5
Table 6. Point scale for ASEPSIS wound inspection score
Proportion of wound affected
0% >0 -19% 20-39% 40-59% 60-79% 80-100%
Serous exudate 0 1 2 3 4 5
Erythema 0 1 2 3 4 5
Purulent exudates 0 2 4 6 8 10
37
Table 7. Interpretation of total ASEPSIS score
ASEPSIS score Meaning
0-10 No infection. Normal healing.
11-20 Disturbance of healing.
21-30 Minor infection
31-40 Moderate infection
41 and over Severe infection
Scoring methods provide more detailed and objective information regarding SSI than CDC and NINSS but they are more costly, complicated and time-
consuming to perform. The average time taken to collect the data and calculate an overall ASEPSIS score is 59 minutes39.
One of the domains for the POMS score is ‘wound infection’. A positive result is defined as ‘wound dehiscence requiring surgical exploration or drainage of pus from operative wound with or without isolation of organisms’. This definition could under-estimate the true wound infection rate. The POMS ‘wound infection’ domain has not previously been compared to other definitions of surgical site infection to assess accuracy.