MRSA screening: can one swab be used for both culture and rapid testing? An evaluation of chromogenic culture and subsequent Hain GenoQuick PCR amplification/detection

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The use of a single swab for both MRSA culture and for rapid testing by PCR was evaluated, using the Hain GenoQuick (GQM) methicillin resistant Staphylococcus aureus (MRSA) assay for the rapid detection of MRSA, as a single swab would be the preferred option for routine diagnostic testing. GQM detected current prevalent Irish MRSA strains incorporating all known SSCmec types, including Panton–Valentine leukocidin-positive strains. Using the GQM method, all methicillin-resistant coagulase-negative staphylococci tested were confirmed to be negative, although three of seven gentamicin-resistant methicillin-sensitive Staphylococcus aureus strains tested were identified as MRSA. The theoretical ex-vivo limit of detection of the assay was 704 CFU per GQM assay reaction (1.7 × 104 CFU/mL) when MRSA suspensions were used for DNA extraction, or 1.4 × 103 CFU/swab (1.4 × 104 CFU/mL) using MRSA absorbed onto Copan screening swabs. Swab processing on chromogenic agar prior to PCR resulted in some inhibition of the PCR reaction, increasing the limit of detection of the assay by a factor of four. Based on 540 single swab screening specimens (nasal and groin) processed first for culture assay, then by GQM, the specificity and positive predictive value were both 100%, the negative predictive value was 92%, and the sensitivity was 57. Culture followed by PCR from a single specimen is not optimal for the rapid detection of MRSA. Further laboratory validation of the GQM assay is required to determine the true diagnostic sensitivity and value of this kit in routine microbiology laboratories, modifying the protocol for single specimens, or using two specimens.