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Methods

D-156 ICAAC Sept 12-15 2010 Boston.

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Methods

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  1. D-156 ICAAC Sept 12-15 2010 Boston MRSA screening: chromID MRSA agar (bioMérieux) vs. BBL CHROMagar MRSA II (BD); selective enrichment vs. non-selective enrichment; 24h incubation vs. 48h.Timothy Vanwynsberghe, Annick Smismans, Johan FransLaboratory of microbiology, Imelda hospital Bonheiden, Belgium Imeldalaan 9 2820 Bonheiden BELGIUM +32015505460 Results Introduction • The aim of the study was to verify whether plating on selective agar after selective enrichment could optimize MRSA detection. • Only GeneXpert (Cepheid) positive samples and samples from decolonization procedures were included. • We compared the following 6 variables: • chromID MRSA agar (bioMérieux) versus BBL CHROMagar MRSA II (BD), • selective enrichment with Brain Heart Infusion + 6.5% NaCl (BD) versus non-selective enrichment with Tryptic Soy Broth (BD), • 24h incubation versus 48h incubation. Ss: sensitivity; Sp: specificity; Pp: positive predictive value; Np: negative predictive value; BM: chromID MRSA agar; BD: BBL CHROMagar MRSA II; BHI: brain heart infusion + 6.5% NaCl; TSB: tryptic soy broth. Best values are in green. • After direct inoculation and 24h incubation approximately 2/3 of the positive samples could be detected; • Enrichment generates an additional sensitivity of 30%; • In most cases the chromID MRSA agar outperforms the BBL CHROMagar MRSA II; • A longer incubation (48h) is only useful with the chromID MRSA agar; • Selective enrichment (BHI + 6.5% NaCl) is only useful with the chromID MRSA agar. Conclusions Methods 101 Screening samples (eSwab, Copan) from nose, throat and perineum were pooled. This pool was stored at -20°C after use. Out of this pool 500µl was inoculated respectively in BHI + 6.5% NaCl and in TSB. At the same time there was direct plating of a chromID agar and a CHROMagar. Each batch contained a positive control (ATCC 43300: MRSA) and a negative control (ATCC 29213: MSSA). All liquid media were incubated for 24 hours after which they were plated on solid media. The direct plated agars were read for the first time and were reincubated. After 48 hours of incubation there was a second reading of the direct plates and a first reading of the enriched plates. After 72 hours these were read for the second and last time. Concordant or discordant results (green on chromID, pink on CHROMagar) were confirmed with the GP and AST-P610 card on VITEK II (bioMérieux) and stored at -80°C. In this study selective enrichment with BHI + 6.5% NaCl and reading until 48h incubation of the chromID MRSA agar was the most sensitive method with the best negative predictive value.

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