Importance of control group selection for evaluating antimicrobial use as a risk factor for methicillin-resistant...

We investigated the importance of control group selection during an evaluation of antimicrobial use as a risk factor for methicillin-resistant Staphylococcus aureus (MRSA) bacteremia at our institution.

We performed a case-control study. A case was defined as any patient admitted between January 1997 and May 2001 who developed nosocomial MRSA bacteremia. We used two control groups; control group I consisted of patients with nosocomial methicillin-susceptible S. aureus (MSSA) bacteremia and control group II included only patients without bacteremia. We matched control-patients to case-patients using age, gender, time at risk, and hospital ward. Data collected on all patients included demographics, comorbidities, antibiotic use, time at risk, length of stay, severity of illness, and outcome.

We evaluated 63 patients (21 in each group). The three groups were well matched regarding age, gender, underlying diseases, and severity of illness. Patients in the MRSA group were more likely to have received a fluoroquinolone and had a higher mean number of days of fluoroquinolone use than did patients in the MSSA group (P = .027 and P = .015, respectively). However, all measures of fluoroquinolone use were similar for case-patients and for control-patients who did not have nosocomial bloodstream infection.

Control group selection is important in evaluating antimicrobial use as a risk factor for MRSA bacteremia. Using control-patients infected with MSSA, rather than uninfected control-patients, may overestimate the association between antimicrobial use and MRSA infection.