Heterogeneous vancomycin-intermediate among methicillin resistant Staphylococcus aureus

BackgroundHetero-resistance vancomycin intermediate Staphylococcus aureus (hVISA) is phenotype, which on in-vitro susceptibility test is vancomycin susceptible (VSSA) but has a minority population of vancomycin intermediate (VISA). hVISA is responsible for vancomycin treatment failure. Population Analysis Profile- Area under Curve (PAP-AUC) is a test for detection of hVISA; however, this test is unsuitable for clinical microbiology laboratory. Tests, such as Brain Heart Infusion Agar with 6 μg/ml vancomycin (BHIA6V), E test and Macromethod E Test (MET) are available; however reported to have variable results.Methods58 clinical isolates of Methicillin resistant S aureus (MRSA) having MIC of vancomycin more than 1 μg/ml by E test and agar dilution were analyzed by PAP-AUC, BHIA6V and MET.ResultThe prevalence of hVISA was 6.9%. hVISA isolates were having vancomycin E test MIC >2 μg/ml. Sensitivity of BHIA6V, MET and E test with MIC >2 μg/ml were 0.75, 0.67 and 1.0 respectively; however, positive predictive values (PPV) were 0.43, 0.4 and 0.27 respectively with PAP-AUC. PAP-AUC ratio correlated with MIC by E test and MET.ConclusionsThere is need for screening MRSA isolates showing in-vitro vancomycin susceptibility ≤2 μg/ml by agar dilution method for detection of hVISA. PAP-AUC test is unsuitable for routine laboratory testing. BHIA6V, MET and E test can be used for screening, however have low PPV.