Methods for screening for methicillin-resistant Staphylococcus aureus carriage

Abstract

Screening patients for methicillin-resistant Staphylococcus aureus (MRSA) carriage at hospital admission is widely accepted as an essential part of MRSA control programmes. It is assumed, although not proven, that rapid reporting of screening results will improve MRSA control, provided that a clear action plan for positive cases is in place and is being followed. An effective culture screening method is direct inoculation of pooled nose, throat and perineal swabs on a well-performing MRSA-selective chromogenic agar; presumptive MRSA colonies can be confirmed rapidly by latex agglutination with antibodies directed against penicillin-binding protein 2a. This method will usually produce a positive result after 24 h of incubation in >95% of true-positive cases, and will be sufficient for most initial treatment and infection control decisions; full antimicrobial susceptibilities will be available on the next day. Inoculation of selective enrichment broth containing a colorimetric growth indicator is an alternative overnight culture method, but there may be problems with overgrowth of other organisms, such as enterococci. PCR methods are now available that can produce same-day results, provided that samples reach the laboratory in time for batch processing, but cultures are required for susceptibility testing. In comparison with culture-based methods, PCR tests are costly, and some have relatively high false-positivity rates; definitive evidence of their clinical cost-effectiveness is lacking. New point-of-care PCR tests are being introduced that are potentially even more rapid but are even more expensive; studies on the clinical cost-effectiveness of these very rapid tests are awaited.