Utility of MRSA nares PCR for non-respiratory cultures in critically ill patients: an observational evaluation

The overuse of antibiotics may lead to complications such as increased resistance, adverse events, and toxicities. Literature demonstrates a negative Methicillin-resistant Staphylococcus aureus (MRSA) nares polymerase chain reaction (PCR) may be used to streamline antibiotic therapy prior to respiratory culture results based on a negative predictive value (NPV) of 95-99%. Additional literature supports a high NPV when MRSA nares PCR is evaluated in non-respiratory cultures; however, this use in critically ill patients has not been studied.

The purpose of this study was to evaluate the clinical utility of MRSA nares PCR in non-respiratory cultures in critically ill patients.

This was a single centre, retrospective, cohort evaluation. Outcomes evaluated were NPV, positive predictive value (PPV), sensitivity, and specificity of MRSA nares PCR in critically ill patients. A sub-group analysis based on the site of culture (blood, urine, and wound) was also conducted.

Of the 325 patients screened, 200 critically ill patients were included for analysis. A total of 259 cultures were evaluated with blood being the most common source (n = 124). The MRSA nares PCR was positive in 34 (17%) patients and thirteen (5%) of the 259 cultures were positive for MRSA. For all cultures, the MRSA nares PCR demonstrated an NPV 99%, PPV 28%, sensitivity 77%, and specificity 85%. The subgroup analysis for the individual culture types reflected similar findings.

A negative MRSA nares PCR may be used to withhold initiation or allow for timely de-escalation of anti-MRSA antibiotics in critically ill patients if clinically applicable.