Comparison of different phenotypic methods of detection of methicillin resistance in Staphylococcus aureus with the molecular detection of mec-A gene

Objective: To evaluate accuracy, cost-effectiveness and ease to perform different phenotypic methods i.e. Cefoxitin 30 ug disc, Oxacillin 1ug disc and Oxacillin agar screening plate (6 ug/ml) for early and accurate identification of MRSA by comparing with the detection of mec-A gene in our clinical isolates. Design: A comparative study. Place and Duration of Study: Clinical samples submitted in the Clinical Microbiology Laboratory at Aga Khan University Hospital, Karachi, from 1^st August to 31^st October 2006. Material and Methods: Out of 200 clinical samples, conventional Polymerase Chain Reaction (PCR) was done on 62 pure biochemically identified S. aureus isolates for mec-A gene detection. Phenotypic methods for detecting methicillin sensitivity (Cefoxitin 30 ug disc, Oxacillin 1 ug disc and Oxacillin agar screening plate) were also used according to the recommended incubation time, duration and temperature on the same isolates. Results: Out of 62 isolates of S. aureus, mec-A gene were detected (MRSA) in 32, whereas 30 were mec-A gene negative (MSSA). Cefoxitin disc and agar screening plate correctly identify all MRSA isolates with the sensitivity and specificity of 100%. Single isolate was false, positively detected as sensitive with Oxacillin 1 ug disc, due to which, the sensitivity and negative predictive value of this method were reduced to 96.9% and 96.8% respectively, while positive predictive value and specificity remained 100%. Conclusion: Comparing different phenotypic methods for MRSA screening in routine microbiology laboratory, Cefoxitin disc and Oxacillin agar screening has better sensitivity and specificity comparative to Oxacillin disc. However, Cefoxitin disc can be preferred especially for small laboratories because it is easy to perform, do not require special technique and media preparation is consequently more cost-effective.