Predicting methicillin resistance among community-onset Staphylococcus aureus bacteremia patients with prior healthcare-associated exposure.

To develop and validate prediction rules to identify the risk of methicillin-resistant Staphylococcus aureus (MRSA) infection among community patients who have healthcare-associated (HA) exposure and S. aureus bacteremia. A total of 1,166 adults with community-onset S. aureus bacteremia were retrospectively enrolled. The background prevalence of community MRSA infection was extrapolated from 392 community-associated S. aureus bacteremia (CA-SAB) patients without HA exposure. Complete and clinical risk scores were derived and tested using data from 774 healthcare-associated S. aureus bacteremia (HA-SAB) patients. The risk scores were modeled with and without incorporating previous microbiological data as a model predictor and stratified patients to low-, intermediate-, and high-risk groups for MRSA infection. The clinical risk score included five independent predictors and the complete risk score included six independent predictors. The clinical and complete risk scores stratified 32.7 % and 42.0 % of HA-SAB patients to the low-risk group for MRSA infection respectively. The prevalence of MRSA infection in score-stratified low-risk groups ranged from 16.3 % to 23.3 %, comparable to that of CA-SAB patients (13.8 %). Simple decision rules allow physicians to stratify the risk of MRSA infection when treating community patients with prior HA exposure and possible S. aureus infection.