Influence of Matching for Exposure Time on Estimates of Attributable Mortality Caused by Nosocomial Bacteremia in Critically Ill Patients

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Abstract
Objective: : To evaluate the influence of matching on exposure time on estimates of attributable mortality of nosocomial bacteremia as assessed by matched cohort studies.Design: : Two retrospective, pairwise-matched (1:2) cohort studies.Setting: : A 54-bed intensive care unit (ICU) in a university hospital.Patients: : Patients with nosocomial Escherichia coli bacteremia (n = 68) and control-patients without nosocomial bacteremia (n = 136 for each matched cohort study).Intervention: : In both matched cohort studies, the same set of bacteremic patients was matched with control-patients using the APACHE II system. In the first study, control-patients were required to have an ICU stay at least as long as the respective bacteremic patient prior to onset of bacteremia (matching on exposure time). In the second study, control-patients were required to have an ICU stay shorter than the stay prior to the development of bacteremia in the respective bacteremic patient (no matching on exposure time).Results: : For bacteremic patients, the mean ICU stay before onset of the bacteremia was 9 days (median, 6 days). In the first matched cohort study, hospital mortality was not different between bacteremic patients and control-patients (44.1% vs 43.4%; P = .999). In the second study, mortality of bacteremic patients and control-patients was also not different (44.1% vs 47.8%; P = .657). Mortality rates between control groups were not different (43.4% vs 47.8%; P = .543).Conclusion: : Matching or not matching on exposure time did not alter the estimate of attributable mortality for ICU patients with E. coli bacteremia.