More accurate measurement of vancomycin minimum inhibitory concentration indicates poor outcomes in meticillin-resistant Staphylococcus aureus bacteraemia

• We evaluated the effect of vancomycin (VAN) on MRSA bacteraemia prognosis.
• A VAN MIC ≥1.5 μg/mL and Charlson co-morbidity index was related to mortality.
• VAN therapy did not affect the clinical prognosis.
• Strains with high a VAN MIC also had high teicoplanin (TEC) and daptomycin (DAP) MICs.
• TEC and DAP may not be used in patients with a high VAN MIC owing to cross-resistance.

Meticillin-resistant Staphylococcus aureus (MRSA) is an important pathogen associated with community-acquired and nosocomial infections. The aim of this study was to validate the vancomycin (VAN) minimum inhibitory concentration (MIC) and administration of VAN that may affect the prognosis of patients with MRSA bacteraemia. In total, 140 clinical MRSA strains from blood cultures were collected from January 2009 to December 2013 at a university hospital in Tokyo (Japan). Patient background, their clinical situation and the susceptibility of isolates to anti-MRSA agents in all cases were reviewed, and factors contributing to 30-day mortality were analysed. Susceptibility to anti-MRSA agents was measured by a microdilution susceptibility testing method. The VAN MIC was further evaluated at 0.25 μg/mL intervals from 0.5 μg/mL to 2.0 μg/mL. Multiple logistic regression analysis revealed a 4-fold increase in mortality of patients with a VAN MIC ≥1.5 μg/mL [odds ratio (OR) = 3.952, 95% confidence interval (CI) 1.471–10.614; P = 0.006]. A one-score increase in the Charlson co-morbidity index resulted in a 1.2-fold increase in the risk of death (OR = 1.199, 95% CI 1.054–1.364; P = 0.006). However, no significant difference was found in the ratio of the VAN 24-h area under the concentration–time curve to MIC between VAN MIC ≥1.5 μg/mL and <1.5 μg/mL. A significant increase in the MICs of teicoplanin and daptomycin was observed in strains with high VAN MICs. For patients with high VAN MICs, administration of these anti-MRSA antibiotics may have a poor outcome owing to cross-resistance.