Introduction and Aims
Methicillin-resistant Staphylococcus aureus (MRSA) has been an increasing problem in hospitals resulting high mortality. Vancomycin (VA) was the first line of treatment. In recent years, there has been an increasing incidence of reduced VA susceptibility, resulting in vancomycin intermediate (VISA) and heterogenous intermediate resistance (hVISA). The aims of this study were to compare the effectiveness of E-test and Vitek in detection of reduced VA susceptibility, performance of VA E-test on 3 Muller Hinton (MH) brands and prevalence of hVISA.
VA susceptibility of 50 MRSA isolates, as confirmed by cefoxitin disc, was determined by VA E-test, Vitek (P-580 AST card) and microbroth dilution (gold standard). The same 0.5 McFarland inoculum was used to perform VA E-test using MH from three different manufacturers, namely BBL, Oxoid and BMX. Detection of hVISA was performed using E-test macromethod using VA and teicoplanin (TP) E-test on brain heart infusion (BHI) agar at Mcfarland 2.0.
Reduced VA susceptibility (MIC = 4 mg/L) were detected in 5 out of the 50 MRSA isolates by microbroth dilution. The number of MRSA isolates with MIC > 2 mg/L as detected by E-test was 10 for BBL and 8 for both Oxoid and BMX MH plates. No MRSA isolates has been detected by Vitek to have MIC > 2 mg/L. The sensitivity for BBL, Oxoid and BMX MH for VA E-test was 100% and specificity for VA E-test was 88.8% for BBL and 93.33% for both Oxoid and BMX. The essential agreement for Oxoid and BMX was 100% (n = 50) and for BBL was 98% (n = 49). No hVISA was detected.
VA E-test has been proven to be more reliable than Vitek in detection of reduced VA susceptibility. Both Oxoid and BMX MH were found to perform better for VA E-test than BBL. Reduced VA susceptibility was found to be not associated with hVISA.