Matching criteria in case-control studies on postoperative infections.

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.