Evaluation of Chromogenic MRSA medium, MRSASelect and Oxacillin Resistance Screening Agar for the Detection of Methicillin Resistant Staphylococcus aureus

Abstract

Aims

An evaluation of Chromogenic MRSA medium (Oxoid) (CMRSA), MRSASelect (MRSAS) (Bio-Rad) and Oxacillin Resistance Screening Agar (Oxoid) (ORSA) was performed to determine the optimum medium providing a rapid and sensitive method for methicillin resistant Staphylococcus aureus (MRSA) detection.

Methods

A total of 632 clinical specimens were cultured onto the three media in first phase of the study, while 720 clinical specimens were cultured on CMRSA and ORSA in the second phase.

Results

The sensitivity and specificity of the media in the first phase were: CMRSA 88.9% & 98.45; MRSAS 92.1% & 99.1%; ORSA (24 hours incubation) 68.3% & 98.8% and ORSA (48 hours incubation) 85.7% & 96.3% respectively. In the second phase the sensitivity and specificity were CMRSA 91.2% & 98.6%; ORSA (24 hours incubation) 58.9% & 98.2% and ORSA (48 hours incubation) 85.6% & 95.6% respectively. The positive predictive values of both the chromogenic media were higher than ORSA. There were fewer false positives with both chromogenic media (1.4% for CMRSA and 0.8% for MRSAS) compared to ORSA (3.3%).

Conclusions

Performing latex agglutination tests on growth from chromogenic media provide results for 93.8% of MRSA isolates within 24 hours. There is a small increase in cost of chromogenic media over ORSA (£28 for MRSAS & £36 for CMRSA per 1000 specimens) and direct agglutination tests (£80 per 1000 specimens). However early availability of MRSA screening results can reduce the burden of MRSA due to early implementation of infection control measures.