Sensitivity and specificity of phenotypic and genotypic tests for detection of methicillin-resistant staphylococcus aureus in clinical isolates

ABSTRACT

Methicillin-resistant Staphylococcus aureus [S. aureus] [MRSA] is an important pathogen in hospitals and communities. There is advance in technologies but infections by MRSA are difficult to diagnose. The current study was conducted to compare three phenotypic methods for with a genotypic method. Cross-sectional study. Methicillin resistance was tested in 450 isolates of S. aureus retrieved from various clinical samples received in Dr. Ziauddin Laboratory Department of Molecular Genetics [DMG], Karachi from January to October 2011. Methods used were, oxacillin disc diffusion, oxacillin screen agar, cefoxitin disc diffusion and Polymerase Chain Reaction [PCR] for mecA gene, which was taken as the gold standard. The Kirby Bauer disc diffusion, using cefoxitin discs 30microg, identified a total of 174 [38.6%] S. aureus isolates as methicillin resistant. With oxacillin disk diffusion test, 152 [33.77%] strains were MRSA with sensitivity of 87.35% and specificity of 100%. Oxacillin-agar screen detected 168 [37.33%] strains with sensitivity of 96.5% and specificity of 100%. PCR method detected mec A gene in 169 [37.55%] isolates with sensitivity of 97.12%, specificity of 100%. The cefoxitin disc diffusion, as suggested by the Clinical and Laboratory Standards Institute [CLSI], is a consistent method for MRSA diagnosis but must be augmented with other methods like oxacillin screen agar, so that no MRSA is overlooked. PCR although taken as the gold standard cannot be recommended as routine because of the high cost and time consumption