B.M. WILLEY, B. TENNANT, T.C. MOORE, L. PEARCE, A. MCGEER, D.E. LOW, M. SKULNICK Mount Sinai Hospital: Toronto, Ontario, Canada

Background: Due to increasing MRSA in Ontario, laboratories have had to improve detection methods and specimen turn around time. Previously, we found the latex agglutination kits Pastorex Staph-plus (Sanofi Diagnostics) and MRSA Screen (Denka Sieken) to accurately identify SA and PBP 2’ MR-protein, respectively. This study validates the use of these kits on colonies picked directly from mannitol salt agar with 2 ¼g /mL oxacillin (MSox2).

Methods: 390 MSox2 growing single yellow colonies from nasal, rectal or wound swabs were studied. Direct "Sanofi" and "Denka" tests were performed by a technologist blinded to routine culture results. After testing, plates were returned for completion of the routine protocol where tests are performed from blood agar (BA) only. All MRSA (Sanofi+, Denka+) identified by direct tests were immediately and only communicated to the infection control officer. Species identification and confirmation of MR were as per standard methods. MRSA were typed by PFGE.

Results: 385/390 (98.7%) concordant results included 107 MRSA, 259 MR-coagulase-negative staphylococci (CoNS), 16 methicillin-susceptible (MS) CoNS and 3 MSSA. 5 discrepant results included 2 MRSA identified by direct but not by routine testing. In each case, MRSA had previously been identified from the patient. 2 MRSA (including a very mucoid strain) were missed by direct testing. Both were Sanofi neg from BA and were only identified as SA by tube coagulase test as per routine protocol. 1MR-CoNS was reproducibly Sanofi+ from the MSox2 leading to its initial misidentification as MRSA. From BA, no agglutination was detectable.

Discussion: Rapid and accurate results were obtained within 24 – 48h of specimen collection when agglutination tests identified MRSA directly from the screen plate. In newly identified cases, this resulted in significantly earlier implementation of infection control precautions (up to 48h).

Presented at:

AMERICAN SOCIETY OF MICROBIOLOGY (ASM) 99^th General Meeting, Chicago, IL, May 30 – June 3, 1999.