Duration of operation as a risk factor for surgical site infection: comparison of English and US data

T times are used to categorize surgical procedures into long and short durations. They constitute a part of the US National Nosocomial Infection Surveillance (NNIS) risk index that is widely used internationally in surveillance for surgical site infections (SSIs). The objective of this study was to compare the US NNIS T times with data collected in England. The Surgical Site Infection Surveillance Service in England holds data collected by 168 hospitals in 13 categories of surgical procedures between 1997 and 2002. The 75^th percentile and corresponding T time were calculated from English data and compared with US times. Differences in rates of SSI above and below the T times were compared. Graphical methods were used to assess the cut points that exhibited an association with risk of SSI. The results show that English and US T times were the same for all surgical categories except coronary artery bypass graft and vascular surgery, where the English T time was 4 h. The 75^th percentile time for hip hemiarthroplasties was 40 min less than for total hip replacements (THR). Although the incidence of SSI in THR was significantly higher in operations lasting for longer than the T time (P < 0.05), no association between risk of SSI and T times set at 1, 1.5 or 2 h was observed for hip hemiarthroplasties. In conclusion, operations lasting for longer than the T time were associated with a higher risk of SSI in most categories. In the hip prosthesis category, this association only applied to THR.