Comparison of two procedures to estimate the hospital stay attributable to nosocomial infection: Matched cohort study versus analysis of covariance of the total unmatched cohort

We compare the results of a matched cohort study with those yielded by analysis of covariance (ANCOVA) in the cohort where the matched study was nested to assess whether the matching design underestimates (as it has been assumed) the extra length of hospitalization due to nosocomial infection. A total of 218 patients developed hospital infection in a cohort of 1483 general surgery patients; 161 were successfully matched 1:1 for surgical procedure, ASA score, age (+/-10 years), emergency-scheduled surgery, preoperative stay, and, whenever possible, number of diagnoses and sex. Unmatched infected patients (57, 23.1%) were different from matched ones. There were no differences for the variables between matched infected patients and their pairs. The matched cohort study overestimates the extra LOH due to hospital infection. The use of ANCOVA in the total cohort obviates the selection bias of the matched cohort design. (C) 1997 Elsevier Science Inc.