To assess the sensitivity of different frequencies of nosocomial infection surveillance (NIS) in general surgery.

Data obtained with a prospective daily NIS are compared with those of hypothetical cross-sectional studies carried out with different frequencies (from one weekly visit up to one visit every other day).

General surgery services at three hospitals.

Sensitivity in the detection of nosocomial infection (overall and stratified by site), compared to a gold standard of prospective surveillance of every patient's complete medical record daily from the first day after surgery until discharge and once more after discharge.

5,859 patients.

837 nosocomial infections were detected by the gold standard (58.8% were surgical-site infections [SSI]). The sensitivity of weekly NIS for all infections was 74.5% (95% confidence interval [CI95], 71.4%-77.5%) and varied from 65.1% (CI95, 56.2%-73.3%) for urinary tract infection to 83.3% (CI95, 62.6%-95.3%) for respiratory tract infection; it was 76.4% (CI95, 72.4%-80.1%) for SSI. As expected, sensitivity increased with the frequency of NIS. Performing NIS every 4 days improved sensitivity significantly, to 82.3% (CI95, 79.5%-84.8%) for all infections and 83.3% (CI95, 79.7%-86.5%) for SSI. One visit every other day increased the sensitivity for all infections by another 4.9%, mainly due to increased detection of urinary tract and other less severe infections.

The sensitivity of two visits a week exceeded that of one weekly visit by approximately 8%, and one visit every other day added another 5% increase. Results varied according to duration of infection and postdischarge hospital stay.