Matching criteria in case–control studies on postoperative infections

Summary Matching is commonly used in case–control studies to control for the effect of major confounding factors. We evaluated the available evidence from case–control studies regarding postoperative infections to identify how frequently matching was performed and with what specific variables. We searched for relevant case–control studies in PubMed until August 2006 and further evaluated those that used individual matching between cases and controls. We identified and evaluated 42 relevant studies. Age was used as a matching criterion in 27 of these 42 (64.3%) case–control studies. The specific type of surgical procedure was the second most commonly used criterion in 17 of 42 studies (40.5%). Gender was used in 14/42 studies (33.3%) as a matching criterion between case and control patients. The period at risk for development of surgical site and/or other postoperative infections, i.e. time from surgery to the diagnosis of infection, was used in nine of 42 studies (21.4%), as was date of operation, and the primary diagnosis that led the case and control patients to surgery. The same surgeon or surgical team was used in seven studies (16.7%); matching according to the National Nosocomial Infection Surveillance system risk score was performed in five studies (11.9%). The findings of our analysis suggest that various characteristics are used for matching in case–control studies of postoperative infections. A more consistent use of matching with the specific type of surgical procedure may help in increasing the internal validity of a case–control study in this field of clinical research.