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Evaluation of four selective media for the detection of methicillin-resistant Staphylococcus aureus from surveillance specimens

Abstract number: o216

Willey  B.M., Kreiswirth  N., Akhavan  P., Tyler  A., Malek  S., Pong-Porter  V., Small  G., Nelson  N., McGeer  A., Poutanen  S.M., Mazzulli  T., Low  D.E., Skulnick  M.

Objectives: 

Rapid, cost effective and practical detection methods from surveillance specimens are integral to continued MRSA control. This blinded study compared a standard-subculture protocol using Oxoid Mannitol Salt Cefoxitin [MSF] agar to direct identification of MRSA from Oxoid Modified-MSF [MOD] agar, Bio-Rad MRSASelect [BR] chromogenic agar, and Becton-Dickinson CHROMagar MRSA [BD].

Methods: 

2500 consecutive swabs (178 wound, 777 rectal, 823 nasals, 713 pooled nasal-axilla-groin-perineum, and 9 other) from 1271 patients in 13 facilities were inoculated onto each medium. MRSA yields, relative workloads and costs were determined at 24 h and 48 h incubation at 37 C. MRSA were confirmed using Pastorex Staph Plus (Bio-Rad), tube coagulase, PBP2a agglutination (Denka Seiken) and CLSI oxacillin screen plate (performed directly from the primary media when possible).

Results: 

147 of the 2500 specimens (5.8%) grew MRSA from at least one medium. Using this as the gold standard, each medium was compared in terms of 24 h/48 h sensitivity, specificity, positive and negative predictive value, % reported, and material and labour costs (expressed as % increase or decrease compared to the MSF protocol).

*Standard-subculture protocol: 18 h and 48 h reads only (no 24 h read) and no direct testing from primary medium

Conclusions: 

The BR read at 24 h performed significantly better (p < 0.00001) than all other media with no added cost; direct testing reduced time to reporting for positive cultures. However, when incubation exceeded 24 h, BR specificity was highly compromised. The MOD, read at 48 h, performed the second best overall and was significantly (p < 0.00001) more specific than MSF. The BD read at 48 h was comparable to the MOD but was limited by mixed growth preventing direct testing thus increasing turn-around-time.

Session Details

Date: 01/08/2007
Time: 00:00-00:00
Session name: XXIst ISTH Congress
Subject:
Location: Oxford, UK
Presentation type:
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