Correlation of methicillin-resistant Staphylococcus aureus vancomycin minimal inhibitory concentration results by Etest and broth microdilution methods with population analysis profile: lack of Etest overestimation of the MIC.

Methicillin-resistant Staphylococcus aureus (MRSA) vancomycin minimum inhibitory concentrations (V-MICs) are sometimes reported to be higher according to Etest versus broth microdilution (BMD). These observations are often interpreted as an Etest overestimation of the actual MIC. We measured V-MIC of 484 MRSA blood isolates using Etest, BMD, and a modified BMD (M-BMD) with incremental dilutions parallel to the Etest scale, correlated the results with population analysis profile-area under the curve (PAP-AUC). All MIC tests were done in parallel. The mean V-MIC was comparable (1.83 ± 0.44 [Etest], 1.88 ± 0.67 [BMD] and 1.75 ± 0.57 mg/L [M-BMD]; p = 0.9 [ANOVA]). The V-MICs/PAP-AUC correlation coefficient was 0.555 (Etest), 0.513 (BMD), and 0.586 (M-BMD). Etest MICs were equal (44.2 %), one dilution higher (21.9 %), two dilutions higher (2.5 %), one dilution lower (29.8 %), and two dilutions lower (1.6 %) than BMD MICs and were equal (61.5 %), one dilution higher (28.3 %), two dilutions higher (0.4 %), one dilution lower (9.5 %), and two dilutions lower (0.2 %) than M-BMD MICs. The mean PAP-AUC for Etest vs M-BMD among isolates with similar Etest/M-BMD MIC values was 0.25 ± 0.15 vs 0.35 ± 0.13 (p = 0.8), 0.46 ± 0.16 vs 0.50 ± 0.17 (p = 0.8), 0.64 ± 0.19 vs 0.67 ± 0.21 (p = 0.9), and 0.90 ± 0.31 vs 0.88 ± 0.25 (p = 1.0) for isolates with V-MIC of ≤ 1, 1.5, 2, and ≥ 3 mg/L respectively. These results suggest that Etest might not overestimate V-MIC in comparison to M-BMD or BMD; Etest and M-BMD tests depict comparable PAP-AUC and have a higher correlation with PAP-AUC than the conventional BMD, probably because of the more detailed results. Etest may be more suitable than conventional BMD for MIC outcome assessment because of the more detailed MICs.

Detection of intermediately vancomycin-susceptible and heterogeneous Staphylococcus aureus isolates: comparison of Etest and Agar screening methods.

Detection of Staphylococcus aureus isolates with intermediate vancomycin susceptibility (VISA) and heteroresistance (hVISA) remains problematic. The population analysis profile/area under the curve (PAP/AUC) is the gold standard but is cumbersome. We compared the performance of two Etest screening methods (macromethod [MAC] and glycopeptide resistance detection [GRD]) plus brain heart infusion (BHI) agars supplemented with 3 (BHI-V3) or 4 (BHI-V4) mg/liter vancomycin in detecting hVISA and/or VISA phenotypes. Etest hVISA screenings were done in parallel for 485 saved methicillin-resistant S. aureus (MRSA) blood isolates according to the manufacturer's instructions. The PAP/AUC was measured for all isolates according to the modified method. PAP/AUC test isolate/Mu3 ratios of <0.9, 0.9 to 1.3, and >1.3 were considered positive for susceptible MRSA (S-MRSA), hVISA, and VISA, respectively. PAP/AUC revealed seven VISA and 33 hVISA phenotypes. MAC screening was positive for 30 (75.0%) hVISA/VISA and 49 (11.0%) S-MRSA isolates. GRD screening was positive for 28 (70.0%) hVISA/VISA and 63 (14.2%) S-MRSA isolates. Growth on BHI-V3 was noted in all hVISA/VISA and 24 (5.4%) S-MRSA isolates. Growth on BHI-V4 was noted in all VISA and four (12.1%) hVISA isolates. None of the S-MRSA isolates grew on BHI-V4 agar. The sensitivity, specificity, and positive (PPV) and negative (NPV) predictive values were 75.0%, 89.0%, 38.0%, and 97.5% for MAC; 70.0%, 85.8%, 30.8%, and 97.0% for GRD; 100%, 94.6%, 62.5%, and 100% for BHI-V3; and 100, 99.2%, 63.6%, and 100% for BHI-V4 (for detecting VISA). These findings suggest that both Etest screening methods have excellent NPV, but positive results require confirmation. BHI-V3 and BHI-V4 agars provide more precise identification of hVISA and VISA, respectively; they may be reasonable alternatives to PAP/AUC.