(530) Evaluation of Gram Stain-Guided Antibiotic Therapy for MRSA Pneumonia in ICU Patients

Introduction: Lower respiratory tract samples are recommended for microbiological evaluation in any patient empirically treated for methicillin resistant Staphylococcus aureus (MRSA) pneumonia. Evidence supporting the efficacy of gram stain (GS) to guide choice of antibiotic therapy in pneumonia has varied, questioning the utility GS has in predicting final culture results. Despite high negative predictive values (NPVs) seen with MRSA nares polymerase chain reaction (PCR) assays, utilization of both respiratory sample (RS) GS and MRSA nares PCR may contribute to overuse of laboratory resources. The purpose of this study was to evaluate if a GS demonstrating no gram-positive organisms from a RS is sufficient to allow for de-escalation of vancomycin therapy and consequently deprioritize the use of MRSA nares PCR.

Methods: This single center study retrospectively identified intensive care unit (ICU) patients started on vancomycin for presumed pneumonia at University of Wisconsin (UW) Health in Madison, WI between August 2022 and March 2023. Patients with no gram-positives on RS GS met inclusion criteria if the RS was ordered within 24 hours of vancomycin being ordered. The primary outcome was NPV of RS GS demonstrating no gram-positive organisms with respect to MRSA detection of the respiratory culture. Secondary outcomes included the NPV of combined MRSA nares PCR plus RS GS, and difference in time to event in patients that had both a RS and MRSA nares PCR ordered within 24 hours of ordering vancomycin.

Results: A total of 371 patients were screened for study eligibility; of which 100 patients met inclusion criteria. NPV of RS GS was 99% for MRSA culture. The combined NPV of RS GS plus MRSA nares PCR was 98.88% for MRSA culture (n = 89). RS was ordered 2.51 hours faster compared to MRSA nares PCR (4.24 vs 6.77 hours, p = 0.021). There was no statistically significant difference between RS and MRSA nares PCR time collected. RS GS resulted 4.69 hours faster compared to MRSA nares PCR (10.66 vs 15.35 hours, p = 0.001).

Conclusions: RS GS was found to result with high NPVs and before MRSA nares PCR. These results support the use of RS GS to de-escalate vancomycin and deprioritize the use of MRSA nares PCR in ICU patients with presumed pneumonia.