Evaluating the Diagnostic Performance of Nasal Methicillin-Resistant Staphylococcus aureus Polymerase Chain Reaction in Hospital-Acquired Pneumonia Within the Intensive Care Unit. A Retrospective Study.

Background: The methicillin-resistant Staphylococcus aureus (MRSA) accounts for 20% to 40% of all hospital-acquired pneumonia (HAP) cases with mortality rates up to 55%. Prompt and accurate diagnosis is essential, especially in intensive care unit (ICU) patients. Nasal MRSA polymerase chain reaction (PCR) diagnostic utility evidence is conflicting in the literature for HAP due to a low number of HAP patients included in prior studies or due to the lack of high-yield gold standard cultures defined for comparisons. Methods: This was a retrospective cohort study conducted in a 65-bed medical ICU, and encompassing all adult patients admitted from January 2015 to March 2023 for HAP. Respiratory cultures included were those obtained by bronchoalveolar lavage or endotracheal suction within 7 days of nasal MRSA PCR testing. Results: The study included 412 patients; 56.8% were males and 65% were Whites. The mean age was 60.5 years. Most patients (82.5%) underwent MRSA-PCR before intubation, and the average time between MRSA-PCR and lower respiratory cultures was 2.15 days. The diagnostic performance of nasal MRSA PCR in diagnosing HAP in the ICU yielded a sensitivity (Sen) of 47.83%, specificity (Sp) of 92.29%, positive predictive value (PPV) of 26.83%, and negative predictive value (NPV) of 96.77%. For nonventilator HAP (nv-HAP) cases sensitivity was at 50%, specificity 92.83%, PPV 28.57%, and NPV at 97.00%. In ventilator-acquired pneumonia (VAP-HAP), the corresponding values were 42.86%, 90.91%, 23.08%, and 96.15%, respectively. Conclusion: The nasal MRSA PCR shows a high NPV and low false negative rate, suggesting it is a reliable tool for ruling out MRSA HAP in ICU patients. Care should be taken into account for disease prevalence and clinical context, as these factors may influence test performance. Further validation through prospective large-sample studies utilizing high-yield lower respiratory tract cultures is necessary to confirm our findings.