A negative nares screen in combination with absence of clinical risk factors can be used to identify patients with very low likelihood of methicillin-resistant Staphylococcus aureus infection in a Veterans Affairs hospital

Background Screening for nares carriage of methicillin-resistant Staphylococcus aureus (MRSA) could be used to identify patients at reduced risk for MRSA infection. However, recent studies suggest that negative results of nares surveillance testing are not reliable to predict a low risk for MRSA infection. Methods To derive a clinical prediction rule, logistic regression was performed to identify predictors of MRSA infection in patients with negative nares surveillance results. To validate the prediction rule and modifications of the rule, the sensitivity, specificity, negative predictive value (NPV), and positive predictive value (PPV) were calculated in a prospective cohort of inpatients receiving empiric vancomycin. Results A prediction rule that included 8 clinical risk factors and nares surveillance results was sensitive with NPV of 99.9%. In the validation cohort, 43 of 451 (10%) patients receiving empiric vancomycin had MRSA infection. In comparison with nares surveillance results (sensitivity, 72%; NPV, 96%), the prediction rule was more reliable for prediction of low risk for MRSA infection (sensitivity, 93%; NPV, 98%). A modified rule that included additional indications for empiric MRSA coverage based on practice guidelines reliably predicted MRSA infection (sensitivity, 100%) while excluding those at low risk (NPV, 100%). Conclusion Negative nares screening results in combination with absence of certain clinical risk factors or syndromes could provide a useful strategy to predict patients with low likelihood of MRSA infection.