Comparison of the BD™ GeneOhm MRSA assay, broth enrichment culture and chromID™ MRSA for detection of methicillin-resistant Staphylococcus aureus from inter-hospital intensive care transfer patients

Abstract

The objective of the present study was to compare the current chromogenic methicillin-resistant Staphylococcus aureus (MRSA) agar plate culture method with broth enrichment and the BD GeneOhm MRSA assay. Another key objective was to determine whether two admission surveillance swabs collected 1 h apart would reliably detect MRSA colonisation in patients transferred to a tertiary referral intensive care unit from other hospitals compared with the current routine method of two samples collected 24 h apart. In total, 593 swabs from 102 consecutive patients transferred from another hospital to the Princess Alexandra Hospital ICU were screened for the MRSA using nose and groin swabs collected on admission, 1 h and 24 h after admission. The non-selective broth enrichment step produced results in complete concordance with the existing method without an increase in sensitivity. The GeneOhm MRSA assay demonstrated 100% sensitivity, 97% specificity and 100% negative predictive values. The initial positive predictive value of this assay, however, was only 28%, largely due to the low prevalence of MRSA. Despite this low positive predictive value and because of the demonstrated 100% negative predictive value, the use of this assay could significantly improve turn-around times of surveillance screens in our laboratory by obviating the need for culture of over 90% of MRSA screening swabs. Positive polymerase chain reaction results require confirmation by culture, given that phenotypical characterisation of isolates is required for infection prevention and control. Comparison of the two screening collection timings in determining MRSA carriage could not be answered, due to insufficient positive results in the study group.