Prevalence and prediction of previously unknown MRSA carriage on admission to a geriatric hospital.

OBJECTIVES to determine the prevalence and characteristics of previously unknown methicillin-resistant Staphylococcus aureus (MRSA) carriers at admission. DESIGN two prospective case-control studies. SUBJECTS 1,621 elderly patients were screened for MRSA carriage within 24 hours after admission to a geriatric hospital in Geneva, Switzerland. METHODS risk factors associated with previously unknown MRSA carriage were determined in the derivation group, and the resulting risk score was evaluated in the validation cohort using logistic regression analysis. RESULTS prevalence of MRSA carriage at admission increased from 7.3% (53/724 patients) in 2001 to 8.7% (78/897 patients) in 2003, with a corresponding prevalence of unknown MRSA carriers of 4.6 and 5.8%, respectively. Three variables were independently associated with previously unknown MRSA carriage: recent antibiotic treatment (adjusted OR (aOR) 2.3; 95% CI 1.0-5.1), intra-hospital transfer (aOR 2.5; 95% CI 1.2-5.3), and hospitalization in the past 2 years (aOR 2.7; 95% CI 1.1-6.7). In the validation cohort, the probability of MRSA carriage increased across risk scores: 0 point, 4% prevalence (6/146); 1 point, 15% (21/136); and $2 points, 31% (21/68; P<0.001). The risk score showed good discrimination and calibration in both groups. CONCLUSIONS our risk score, which used a simple additive point system to estimate the likelihood of unknown MRSA carriage, had good accuracy and generalised well in an independent sample of patients. Once validated in a clinical trial, our risk score may be used as a tool to optimise MRSA control.