The Society for Healthcare Epidemiology of America (SHEA) has strongly recommended active surveillance cultures for methicillin-resistant Staphylococcus aureus (MRSA) as a key strategy for the control of healthcare-associated infections (HAIs).

A recently published article in the Journal of Clinical Microbiology¹ compared the results of BBL CHROMagar MRSA (BD Diagnostics, Sparks, MD) to a traditional algorithm for MRSA detection. In addition, the results of BBL CHROMagar MRSA were compared to five different susceptibility testing methods using S. aureus strains isolated on Trypticase Soy Agar with 5% Sheep Blood Agar (TSA II). The study revealed that BBL CHROMagar MRSA provided improved recovery, specificity, turnaround time, labor savings, and workflow as compared to these other methods. BBL™ CHROMagar™ MRSA was cleared by the FDA in January 2005 as a selective and differential medium for the qualitative direct detection of nasal colonization by MRSA.

From 2,015 surveillance specimens, 354 S. aureus isolates were recovered: 208 (59%) were oxacillin (methicillin) susceptible and 146 (41%) were oxacillin resistant. BBL CHROMagar MRSA detected 139 of 146 MRSA isolates for a 95.2% recovery rate and TSA II recovered 127 of 146 or 86.9% . The difference in rates of recovery from the two media was statistically significant. For BBL CHROMagar MRSA, 86% of MRSA isolates were recovered at 24 hours and the remaining 14% were recovered at 48 hours. The specificity of BBL CHROMagar MRSA was 99.7% (1,863/1,869) at 24 hours. At 48h, 68 cultures demonstrated mauve colonies that were not MRSA isolates. By excluding mauve colonies that were coagulase negative or had a Gram stain not consistent with MRSA at 48 hours, the overall specificity of BBL CHROMagar MRSA remained at 99.7%.

The authors noted significant benefits gained by using BBL CHROMagar MRSA that include identification of most MRSA isolates at 24 hours without additional susceptibility testing, enhanced recovery of MRSA, and suppression of methicillin-susceptible Staphylococcus aureus (MSSA) and other non-MRSA species that might be present in the nose. These benefits contributed to substantial labor and material cost savings. Two hundred eight (208) of 354 (59%) S. aureus isolates in the study were MSSA that were inhibited on BBL CHROMagar MRSA. The authors stated "From these data from routine clinical specimens, it is apparent that use of BBL CHROMagar MRSA would result in a significant reduction in the numbers of coagulase and susceptibility tests performed in a clinical laboratory."

The authors concluded "As laboratories rise to help meet the challenge of control of MRSA in the healthcare facilities and now in the community, rapid diagnostic methods are needed. For many, BBL CHROMagar MRSA can fulfill this need. CHROMagar MRSA was superior to TSA II for recovery of MRSA from swab surveillance cultures of the anterior nares, and it may shorten the time to detection of MRSA by 24 to 48 hours, depending upon the methods used."

Reference:
¹ Diane Flayhart et al., Multicenter Evaluation of BBL CHROMagar MRSA Medium for Direct Detection of Methicillin-Resistant Staphylococcus aureus from Surveillance Cultures of the Anterior Nares, Journal of Clinical Microbiology, November 2005, p. 5536-5540, Vol. 43, No. 11