Comparison of culture screening protocols for methicillin-resistant Staphylococcus aureus (MRSA) using a chromogenic agar (MRSA-Select).

To compare the culture screening protocols for methicillin-resistant Staphylococcus aureus (MRSA), a total of 300 duplicate nasal swabs (233 initial cultures and 67 weekly follow-up cultures) were collected consecutively from 233 patients in the Intensive Care Unit (ICU). One swab was plated directly on MRSA-Select agar (D-MRSA-Select) and observed at 24 hr. The duplicate swab was incubated in tryptic soy broth (TSB) with 6.5% NaCl for 24 hr, and then subcultured on MRSA-Select (B-MRSA-Select), BAP (B-BAP), and mannitol salt agar with 4 mg/L oxacillin (B-MSA(OXA)), and observed at 24 hr. MRSA was detected in 13.7% (32/233) of the initial and 22.4% (15/67) of the follow-up specimens. A patient was classified as MRSA-positive if any of the media grew colonies that were tested and confirmed to be MRSA. In the initial screening samples, the sensitivities of D-MRSA-Select, B-MRSA-Select, B-BAP, and B-MSA(OXA) were 78.1%, 84.4%, 78.1%, and 65.6%, respectively, and the specificities were 100%, 98.0%, 83.1%, and 93.5%, respectively. The sensitivities of all but the B-MRSA-Select protocol were significantly lower (p <0.05). In follow-up screening, the sensitivities of D-MRSA-Select, B-MRSA-Select, B-BAP, and B-MSA(OXA) were 66.7%, 86.7%, 66.7%, and 53.3%, respectively, and the specificities were 100%, 98.1%, 90.4%, and 90.4%, respectively. D-MRSA-Select protocol was considered useful in screening for MRSA because it was fast, highly specific, and showed sensitivity comparable to B-BAP. Salt-containing enrichment broth in conjunction with MRSA-Select (B-MRSA-Select) provides a promising way to increase sensitivity in initial and follow-up screening for MRSA.