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Objective:To assess the sensitivity of different frequencies of nosocomial infection surveillance (NIS) in general surgery.Design: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).Setting:General surgery services at three hospitals.Main Outcome Measure: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.Patients:5,859 patients.Results: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.Conclusions: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.