Evaluation of a Chromogenic Agar (MRSASelect) for the Detection of Meticillin-resistant Staphylococcus Aureus with Clinical Samples in The Netherlands

A novel chromogenic medium for the detection of meticillin-resistant Staphylococcus aureus (MRSA), MRSASelect (Bio-Rad), was evaluated with clinical samples in a public health laboratory in The Netherlands. In total, 3000 samples were tested in the period January to March 2005, including 972 nose, 972 throat, 968 perineum, and 88 wound or urine samples. Presumptive MRSA colonies appeared pink/mauve on the MRSASelect medium. The performance of MRSASelect medium was compared with the routine screening method. Evaluation of the colony morphology showed that all confirmed MRSA isolates grew as pink/mauve colonies. None of the white colonies were MRSA strains. The number of false-positive pink/mauve colonies increased after prolonged incubation from 20 to 48 h. The specificity decreased from 92 % after 20 h incubation to 89 % after 48 h incubation. In total 70 MRSA strains were isolated, 55 of which were detected by the MRSASelect medium and 55 were detected by the routine screening method. Sensitivity was 78.6 % for both test procedures, and specificities were 99.5 and 100 %, respectively for the MRSASelect medium and the routine screening method. The addition of an enrichment broth to the MRSASelect medium increased the number of MRSA strains detected by 12 %. In total, 18 patients were MRSA positive, 4 of these were detected by the MRSASelect medium only and 1 was detected by the routine screening method only. Sensitivity on patient level was 94.4 and 77.8 % for the MRSASelect medium and the routine screening method, respectively, while specificities were 99.7 and 99.0 %.