To examine the effect of heterogeneous case mix for a benchmarking analysis and interhospital comparison of the prevalence rates of nosocomial infection.

Cross-sectional survey.

Eleven hospitals located in Cyprus and in the region of Crete in Greece.

The survey included all inpatients in the medical, surgical, pediatric, and gynecology-obstetrics wards, as well as those in intensive care units. Centers for Disease Control and Prevention criteria were used to define nosocomial infection. The information collected for all patients included demographic characteristics, primary admission diagnosis, Karnofsky functional status index, Charlson comorbidity index, McCabe- Jackson severity of illness classification, use of antibiotics, and prior exposures to medical and surgical risk factors. Outcome data were also recorded for all patients. Case mix-adjusted rates were calculated by using a multivariate logistic regression model for nosocomial infection risk and an indirect standardization method.

The overall prevalence rate of nosocomial infection was 7.0% (95% confidence interval, 5.9%–8.3%) among 1,832 screened patients. Significant variation in nosocomial infection rates was observed across hospitals (range, 2.2%–9.6%). Logistic regression analysis indicated that the mean predicted risk of nosocomial infection across hospitals ranged from 3.7% to 10.3%, suggesting considerable variation in patient risk. Case mix-adjusted rates ranged from 2.6% to 12.4%, and the relative ranking of hospitals was affected by case-mix adjustment in 8 cases (72.8%). Nosocomial infection was significantly and independently associated with mortality (adjusted odds ratio, 3.6 [95% confidence interval, 2.1–6.1]).

The first attempt to rank the risk of nosocomial infection in these regions demonstrated the importance of accounting for heterogeneous case mix before attempting interhospital comparisons.