Relationship between the MIC of vancomycin and clinical outcome in patients with MRSA nosocomial pneumonia

This retrospective cohort study involved adults with MRSA nosocomial pneumonia treated with vancomycin. Vancomycin MICs were determined using Etest. Patients with MRSA and vancomycin MICs ≥1.5 μg/mL and those with MRSA and MICs ≤1 μg/mL were placed in the high- and low-MIC group, respectively. The primary outcomes assessed were clinical response and relapse of MRSA pneumonia within 28 days after vancomycin discontinuation. Secondary outcomes included 28-day mortality, in-hospital mortality and length of hospital stay.

Seventy patients met the inclusion criteria. Mean age and mean Acute Physiological and Chronic Health Evaluation (APACHE) II score upon intensive care unit (ICU) admission of these patients were 67.0 years and 25.9, respectively. Thirty-four (48.6%) isolates had high vancomycin MICs, and 36 (51.4%) had low MICs. There were no significant differences in baseline characteristics between the two groups. Early clinical response rates in the low- and high-MIC groups were 63.9% and 35.3%, respectively (p = 0.031). The high-MIC group had an 8% lower final clinical response rate, but this difference was not significant (p = 0.609). The relapse rate within 28 days was significantly higher in the high-MIC group than in the low-MIC group (29.6% versus 6.9%, p = 0.038). On multivariate analysis, infection by high-MIC strains was an independent predictor of early clinical response failure.

About half of the MRSA isolates had high vancomycin MIC. Patients infected with these strains showed slower clinical response and higher relapse rate than patients infected with low vancomycin MIC isolates.