Comparative evaluation of the Phoenix®, VITEK® 2, E-test® and microdilution test for vancomycin susceptibility testing in Staphylococcus aureus isolated from bloodstream infection

Abstract

Introduction: Staphylococcus aureus bacteremia causes significant morbidity and mortality, mainly by methicillin-resistant S. aureus (MRSA). Currently, vancomycin is the main choice for the treatment of infections by MRSA. Broth microdilution (BMD) remains the gold standard for measuring vancomycin MIC. However, most clinical laboratories employ practical methods in the routines, but these methods may not determine accurate vancomycin MIC values. Objectives: This study aimed to evaluate the accuracy of VITEK®2, Phoenix® and Etest® methods against BMD. Materials and Methods: A total of 78 strains (27 methicillin-sensitive S. aureus and 51 MRSA) were isolated from bloodstream infections. The vancomycin MIC was determined following CLSI and the manufacturers' recommendations. We also performed SCCmec typing, in order to identify their vancomycin MIC ratio values. Results: Most of all isolates showed values of MIC = 1 μg/mL by BMD and Phoenix®, while Etest® and VITEK® 2 determined the majority with MIC = 1.5 and 0.5 μg/mL, respectively. Thus, Etest® and VITEK® 2 tended to overestimate and underestimate, respectively, the MIC values. Three MRSA isolates that were vancomycin susceptible by the BMD were vancomycin-intermediate by Etest®. The SCCmec II (39%) and IV (51%) were the most frequent, and there was no relationship between the type of SCCmec and the MIC values. Conclusions: The results showed that vancomycin MICs vary according to the test method. It is essential that clinicians consider the differences in MIC results determined by different methods, since the MIC value is generally the parameter used by clinicians to select the appropriate therapy.