Relationship of Vancomycin Minimum Inhibitory Concentration and Treatment Outcomes among Patient with Methicillin-Resistant Staphylococcus aureus Bacteremia: A Four Year Retrospective Study

Abstract

Introduction: Therapeutic failures with vancomycin in Methicillin-resistant Staphylococcus aureus (MRSA) infections have been increasingly reported. There is growing evidence that infection with an organism with an increased vancomycin minimum inhibitory concentration (MIC) that is still within the susceptible range may also be associated with increased rates of vancomycin failure. This study aimed to determine the association between vancomycin MIC and treatment failures among patient with MRSA bloodstream infection in Thailand. Methods: This retrospective study was conducted at Sunpasitthiprasong hospital, a tertiary care and academic hospital. All medical records of the patients who had MRSA bacteremia and were treated with vancomycin between January 2012 and 2015 were reviewed. Patients were categorized into two groups; MIC≤1 mg/L (low) and MIC>1 mg/L (high). The primary outcome was overall failure (thirty-day mortality, microbiological failure and recurrence of MRSA infection). Results: During the study period, 121 patients met the inclusion criteria. Of these, 50 (41.3%) and 71 (58.7%) patients had vancomycin MIC ≤ 1 mg/L and MIC > 1 mg/L respectively. Demographic data were generally similar between the two groups. Overall failure was statistically significantly different between low and high vancomycin MIC with p-value <0.001. Twenty seven patients (22.3%) in both groups had thirty day mortality. Of these, 5 (10%) patients were in the low vancomycin MIC group and 22 (31%) patients were in the high vancomycin MICs group. There was a significant difference (OR 4.04, 95%CI: 1.33-14.67, p-value 0.008). Microbiological failure and recurrence MRSA infection were also high and consistent with thirty day mortality in high vancomycin MICs group. Approximately 65% of all patients achieved vancomycin level at steady state of at least 15 mg/L and there were no differences of trough vancomycin level between two groups. Conclusion: The present study strongly suggests that patients with MRSA bloodstream infections with vancomycin MICs of >1.0 mg/L respond poorly to vancomycin therapy and were significantly associated with overall failure.