title: 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.
abstract: 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.
authors: Department of Internal Medicine, St John Hospital and Medical Center, 19251 Mack Avenue; suite 340, Medical Education, Grosse Pointe Woods, MI 48236, USA. riad.khatib@stjohn.org
authors: R
authors: R
authors: Khatib
authors: K
authors: K
authors: Riederer
authors: S
authors: S
authors: Shemes
authors: A C
authors: AC
authors: Musta
authors: S
authors: S
authors: Szpunar
mesh_terms: D000368:Aged; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D019540:Area Under Curve; D006801:Humans; D055624:Methicillin-Resistant Staphylococcus aureus / Q000187:drug effects* / Q000302:isolation & purification; D008826:Microbial Sensitivity Tests / Q000379:methods*; D008875:Middle Aged; D013203:Staphylococcal Infections / Q000382:microbiology*; D014640:Vancomycin / Q000494:pharmacology*
chemical_list: D000900:Anti-Bacterial Agents; D014640:Vancomycin