Surgical-site infections and the NNIS SSI Risk Index: room for improvement.

Surveillance of surgical-site infections (SSI), with feedback of appropriate data to surgeons, has been shown to be an important component of strategies to reduce SSI risk.1-3 For SSIs, the traditional wound classification system, which stratifies each wound into one of four categories (clean, clean-contaminated, contaminated, and dirty-infected), has been available since 1964.2-4 Limitations of this system of risk stratification are well recognized. One of the major problems is its failure to account for the intrinsic patient risk of developing an SSI. A composite risk index that captures the joint influence of this and other risk factors is required before meaningful comparisons of SSI rates can be made among surgeons, among institutions, or across time. A simple index was developed during the Study on the Efficacy of Nosocomial Infection Control Project.5 Since 1991, a modification of this risk index has been used by National Nosocomial Infection Surveillance (NNIS) System hospitals.6 The NNIS Basic SSI Risk Index is a significantly better predictor of SSI risk than is the traditional wound classification system and performs well across a broad range of operative procedures. Initially, it seemed most attractive to collect and analyze the same few data fields for risk adjustment in calculation of SSI rates, primarily to decrease the data-collection burden and add simplicity to the interpretation of the rates. In fact, it was surprising that the same few risk factors stratified better than wound class alone for so many diverse procedures. The NNIS Basic SSI Risk Index performed reasonably well for all but a handful of procedures. Ah, but this simplicity could not last. The last decade has witnessed changes to healthcare delivery with regard to surgical procedures. Considerable numbers of procedures are now performed on outpatients, and the surgical patients admitted to hospitals tend to have higher intrinsic risk and often are discharged earlier.7-9 Roy and colleagues report in this issue of the Journal a simple matched case-control study of 201 cardiothoracic patients who developed an SSI and 398 controls who did not (two controls per case matched on age, gender, type of procedure, and date of procedure). 10 Two main results that the authors found were equally simple to describe and were totally consistent with previously published NNIS results, which were based on a cohort of patients many times larger and coming from many hospitals: ● For coronary artery bypass graft (CABG) procedures, the NNIS Basic SSI Risk Index is largely just a dichotomization of patients based on the cut point for duration of surgery, because almost all of these procedures are clean and done on patients with anesthesiology scores of 3 or 4. ● The SSI rate is significantly higher in patients with a risk index score 2 compared with patients having a risk index score <2. The authors found an odds ratio of 1.83 (P=.01), which is nearly identical to the odds ratio of 1.75 in NNIS-published data.11 Thus, the NNIS Basic SSI Risk Index does stratify patients undergoing CABG operations by their risk of SSIs, as evidenced by the authors’ own data. A major point of the authors’ manuscript, that for CABG, the elements of the NNIS Basic SSI Risk Index may not have equal weights, is valid but not new.11 This criticism is inconsequential, though, compared with the issue at hand. The NNIS Basic SSI Risk Index could be improved for CABG and many other procedures as well. Efforts are needed to use available NNIS data at the Centers for Disease Control and Prevention to improve the NNIS Basic SSI Risk Index. Recent NNIS analyses from the 4th Decennial International Conference on Nosocomial Infections for cholecystectomy, gastric surgery,