Converting incidence and prevalence data of nosocomial infections: Resultsfrom eight hospitals

OBJECTIVE: To investigate the use of the formula of Rhame and Sudderth for the interconversion of prevalence and incidence data on the frequency of no socomial infections. DESIGN: Comparison of observed and calculated incidence data and prevalence data. SETTING: One 8-week incidence investigation in the surgical and intensive c are units of eight medium-sized hospitals; three separate point-prevalence studies in the same units. RESULTS: The overall prevalence observed after the three prevalence studies in 2,169 patients was 6.8% (95% confidence interval [CI95], 5.7-8.0). In 2 ,882 discharged patients observed during the incidence study, the mean hosp italization was 9.8 days; patients with one or more nosocomial infection ha d a mean hospitalization time of 22.3 days and a mean interval of 8.2 days from admission to the brat day of infection. Based on these data, the overa ll calculated incidence was 4.7%, whereas the observed incidence was 4.3% ( CI95, 3.6-5.2). Vice versa, an overall prevalence of 6.2% was found when ca lculated from the observed incidence data. The incidence data calculated from prevalence data also were within the con fidence interval of the incidences observed for urinary tract infections an d surgical-site infections. (However, it was not possible to convert the da ta for two of the eight hospitals.) CONCLUSION: The approximate mathematical relationship between the prevalenc e and incidence data of nosocomial infection is confirmed by this study. Ho wever, although it is theoretically possible, we would not recommend the co nversion of prevalence into incidence data or vice versa (Infect Control Ho sp Epidemiol 2001;22:31-34).