Does sub-culturing of positive MRSA blood cultures affect vancomycin MICs?

. Empirical vancomycin (VAN) treatment failure for methicillin-resistant (MRSA) bacteraemia, with significantly higher mortality, has been reported for MRSA strains with reduced VAN susceptibility.. Our goal was to study the effect of sub-culture on VAN minimum inhibitory concentration (MIC) values compared to direct susceptibility of MRSA-positive blood cultures.. Using 19 MRSA-positive blood cultures and 19 seeded MRSA-positive blood cultures, we compared the VAN MICs from direct susceptibility testing of MRSA-positive blood cultures and MRSA sub-cultured from positive blood cultures.. In comparing direct VAN MICs from MRSA-positive blood cultures and standard agar dilution, nearly half of the MICs from agar dilution were lower, with one sample decreasing from 1.5 to 0.75 µg ml. Furthermore, in seeded blood cultures, 80 % or more showed lower values from standard agar dilution compared to direct VAN MICs.. Our results reveal a trend towards lower MICs after positive blood culture isolates are sub-cultured. Some clinical failures among MRSA infections treated with VAN may result from this phenomenon.