Screening for Vancomycin Intermediately Susceptible and Heteroresistant Staphylococcus aureus using Vancomycin-Supplemented Brain-Heart Infusion Agar Biplates: Defining Growth Interpretation Criteria based on Gold Standard Confirmation.
J Clin Microbiol. 2015 Aug 26;
Authors: Khatib R, Riederer K, Sharma M, Shemes S, Iyer SP, Szpunar S
BHI agar supplemented with vancomycin 4 (BHI-V4) and 3 mg/L (BHI-V3) have been proposed for screening vancomycin intermediately-susceptible Staphylococcus aureus (VISA) and heteroresistant (hVISA) phenotypes respectively, but growth interpretation criteria have not been established. We reviewed the growth results (CFU) during population analysis profile-area under the curve (PAP-AUC) of consecutive methicillin-resistant Staphylococcus aureus (MRSA) blood isolates, saved intermittently between 1996 and 2012. CFU counts on BHI-V4 and BHI-V3 plates were stratified according to PAP-AUC interpretive criteria: <0.90 (susceptible [S-MRSA]), 0.90-1.3 (hVISA), >1.3 (VISA). CFU cutoffs that best predict VISA and hVISA were determined with the use of receiver operating characteristic (ROC) curves. Mu3, Mu50 and MSSA controls were included. We also prospectively evaluated manufacturer-made BHI-V3/BHI-V4 biplates for screening of 2010-2012 isolates. The PAP-AUC of 616 clinical samples was consistent with S-MRSA, hVISA and VISA in 550 (89.3%), 48 (7.8%) and 18 (2.9%) instances, respectively. For VISA screening on BHI-V4, a cutoff of two CFU/droplet provided 100% sensitivity and 97.7% specificity. To distinguish VISA from hVISA, a cutoff of 16 CFU provided 83.3% sensitivity and 94.7% specificity; the specificity was lowered to 89.5% with a 12 CFU cutoff. For detecting hVISA/VISA on BHI-V3, a two CFU/droplet cutoff provided 98.5% sensitivity and 93.8%, specificity. These results suggest that two CFU/droplet cutoffs on BHI-V4 and BHI-V3 best approximate VISA and hVISA gold standard confirmation respectively with minimal overlap in samples with borderline PAP-AUC. Simultaneous screening for VISA/hVISA on manufacturer-made BHI-V4/BHI-V3 biplates is easy to standardize and may reduce the requirement for PAP-AUC confirmation.
PMID: 26311860 [PubMed - as supplied by publisher]