Evaluation of separate vs pooled swab cultures, different media, broth enrichment and anatomical sites of screening for the detection of methicillin-resistant Staphylococcus aureus from clinical specimens.

Early identification of methicillin-resistant Staphylococcus aureus (MRSA) carriers is a major component of an MRSA control programme. The cost and laboratory workload could be markedly reduced by processing multiple swabs from one person in one culture broth (specimen pooling). We evaluated the sensitivity for MRSA detection and the growth rate of pooled swabs compared with individual processing. In total, 1254 swabs from 423 subjects (two to five swabs per subject) were submitted for detection of MRSA. Swabs were suspended in 2-mL volumes of sterile Todd-Hewitt Broth and divided into two 1-mL aliquots. One aliquot of the suspension was processed as a single specimen, and the other aliquot was mixed (pooled) with other suspensions in which swabs from the same patient were suspended. Forty-four (10%) pooled samples were positive for MRSA. Specimens from seven additional patients that were negative when pooled were positive when processed separately. There was no case where the pooled specimen was positive but the separate specimens were negative. The diagnostic sensitivity of pooled surveillance cultures compared with single cultures, when only subjects colonized by MRSA were considered, was 86% and the false-negative rate was 14%. Eighty percent of the pooled positive cultures were detected by the third day and all were detected by the fourth day. Fifty-four percent of the specimens processed separately were detected by the second day and all were detected by the fourth day. Pooling of specimens decreases the sensitivity of MRSA detection compared with processing each swab separately, particularly in swabs with a low number of colony-forming units. In all subjects whose pooled samples were negative but whose swabs examined separately were positive, the swabs examined separately were negative on primary plates and positive only after culturing in enrichment broth.