Detection of Staphylococci with Reduced Susceptibility to Glycopeptides

As pointed out by Dr. Walsh and colleagues in the July 2001 issue of the Journal of Clinical Microbiology (4), the detection of staphylococci with reduced susceptibility to vancomycin is an important issue for clinical laboratories. In their study, Dr. Walsh et al. evaluated several methods for detection of these strains using a population-analysis profile-area under the curve ratio method as their “gold standard.” Two NCCLS reference methods, broth microdilution and agar dilution, were included in their analysis. They state that both were performed “precisely” as described in the most recent NCCLS document, M7-A5 (2, 3). However, the incubation time for both tests in their study was only 18 h, whereas current NCCLS recommendations specify that tests for both oxacillin and vancomycin should be incubated for a full 24 h. We suspect that the sensitivity of both reference methods would have improved had the readings been made at 24 h rather than at 18 h as was done in their study. Incubation for a full 24 h appears to be important not just for NCCLS reference microdilution susceptibility tests but also for primary isolation plates (1), as pointed out in a study by other investigators in the same issue of the journal.