MRSA nare test has high NPV


The main clinical value of the methicillin-resistant Staphylococcus aureus (MRSA) nares test is when it yields a negative result. The evidence consistently shows an excellent negative predictive value (NPV) that ranges between 95% to 98% depending on the type of pneumonia, whether hospital-acquired or community-acquired pneumonia (HAP or CAP, respectively) (choice D is correct). The MRSA nares test has a poor positive predictive value (PPV) for MRSA pneumonia (range between 36%-57%), and is therefore not of great utility when the results are positive (choice C is incorrect). The sensitivity of MRSA nares test is variable with highest numbers (85%) for patients with CAP and lowest for patients with ventilator-associated pneumonia (VAP, 40%) (choice A is incorrect). The specificity ranges from 90% to 94% for patients with pneumonia (choice C is incorrect).

The actual characteristics of the MRSA nares test have been assessed in a systematic review that showed the pooled prevalence of potential MRSA pneumonia was 10%. The pooled sensitivity, specificity, PPV, and NPV of MRSA nares screen for all MRSA pneumonia types were 71%, 90%, 45%, and 96.5%, respectively. Patients, as suggested in this case, had better sensitivity, specificity, PPV and NPV (85%, 92%, 57%, and 98%, respectively) for MRSA nares screen (HAP or CAP) compared with patients with VAP (40%, 94%, 36% and 95%, respectively). In this question the prevalence of the disease is not mentioned, as it is unlikely that there will be a known prevalence for each particular unit, hospital and geographic location. Accordingly, using the baseline 10% prevalence used in the systematic review and the multiple prevalence reports of MRSA in the setting of microbiology tested CAP, it will be unlikely to have a prevalence above 10%. Even in those circumstances of unusually high prevalence rates it may affect mildly the NPV and it is possible to remain above 95%. Therefore, these data suggest that MRSA nasal screening is a valuable tool to rule out MRSA pneumonia, particularly in cases of CAP.1

Footnotes

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