Interpretation of MRSA Select Screening Agar at 24 Hours of Incubation

Abstract

An incubation time of 24 h at 35 to 38°C is recommended for the optimal performance of MRSA Select (Bio-Rad) chromogenic screening agar. An additional 24 h is required to capture slow-growing methicillin-resistant Staphylococcus aureus (MRSA). However, the normal hours of operation for most laboratories cannot reliably accommodate 24-h interpretation intervals. Daily agar plate interpretations are more likely to occur around 18 h and 42 h of incubation, which may compromise the performance of the chromogenic agar and negatively impact patient infection control efforts. In order to validate the importance of stringent incubation times to plate performance, we evaluated MRSA Select medium at controlled intervals of 24 and 48 h of incubation, using clinical MRSA-screening swabs. A total of 1,071 MRSA-positive and 2,733 MRSA-negative cultures were selected for analysis. Compared to 48-h-incubation results, the sensitivity and specificity of MRSA Select at 24 h were 98.3% and 98.2%, respectively. Only 19 of 1,071 (1.8%) MRSA-positive isolates required 48 h for detection. Holding 24-h negative plates an additional 24 h resulted in the workup of 253 (6.6%) pink, yet non-MRSA, colonies. The 24-h positive and negative predictive values of MRSA Select , assuming MRSA prevalences of 1, 5, and 10%, were 35.5 and 99.98%, 74.2 and 99.9%, and 85.9 and 99.8%, respectively. MRSA Select medium held for 24-h incubation is a highly sensitive and specific MRSA-screening tool. Further incubation prolongs the turnaround time for results and creates a significant amount of unnecessary work in the laboratory.