Performance of the National Nosocomial Infections Surveillance risk index in predicting surgical site infection in Australia

Background. The Victorian Hospital Acquired Infection Surveillance System (VICNISS) hospital-acquired infection surveillance system was established in 2002 in Victoria, Australia, and collates surgical site infection (SSI) surveillance data from public hospitals in Australia. Objective. To evaluate the association between the US National Nosocomial Infections Surveillance (NNIS) system's risk index and SSI rates for 7 surgical procedures. Methods. SSI surveillance was performed with NNIS definitions and methods for surgical procedures performed between November 2002 and September 2004. Correlations were assessed using the Goodman-Kruskal gamma statistic. Results. Data were submitted for the following numbers of procedures: appendectomy, 545; coronary artery bypass graft (CABG), 4,632; cholecystectomy, 1,001; colon surgery, 623; cesarean section, 4,857; hip arthroplasty, 3,825; and knee arthroplasty, 2,416. NNIS risk index and increasing SSI rate were moderately well correlated for appendectomy (gamma=0.55), colon surgery (gamma=0.48), and cesarean section (gamma=0.42). A fairly positive correlation was found for cholecystectomy (gamma=0.17), hip arthroplasty (gamma=0.2), and knee arthroplasty (gamma=0.16). However, for CABG surgery, a poor association was found (gamma=0.02). Conclusions. The NNIS risk index was positively correlated with an increasing SSI rate for all 7 procedures; the strongest correlation was found for appendectomy, cesarean section, and colon surgery, and the poorest correlation was found for CABG surgery. We believe that risk stratification with the NNIS risk index is appropriate for comparison of data for most procedures and superior to use of no risk adjustment. However, for some procedures, particularly CABG, further studies of alternative risk indexes are needed to better stratify patients.