Evaluation of vancomycin MIC creep in Staphylococcus aureus

Abstract

Objectives: Vancomycin is the primary treatment for methicillin-resistant Staphylococcus aureus (MRSA). However, an increasing proportion of MRSA isolates with high minimum inhibitory concentrations (MICs) within the susceptible range (vancomycin 'MIC creep') is being observed. The aim of this study was to assess the vancomycin MIC distribution for S. aureus isolates over a period of 4 years in Centro Hospitalar Baixo Vouga (Aveiro, Portugal) and to identify differences in vancomycin MIC determined by different susceptibility testing methods.

Methods: For each S. aureus isolate, the vancomycin MIC was assayed by the VITEK^®2 automated system and the broth microdilution testing method.

Results: The results showed significant differences in vancomycin MIC by different methods (P=0.021, sign test) and did not suggest the presence of vancomycin MIC creep during the study period.

Conclusions: Vancomycin MIC creep is a regional problem, therefore it can only be assessed through the evaluation of local susceptibility profiles, and antibiogram based on real MIC assay should be an essential element in local MRSA infection clinical management.

Keywords: MIC creep; Staphylococcus aureus; Susceptibility testing methods; Vancomycin.