Evaluation of Five Different Phenotypic Methods for Detection of Methicillin Resistance in Coagulase-Negative Staphylococci (CoNS) Isolated from Clinical Specimens

Rapid and accurate detection of methicillin resistant coagulase -negative staphylococci (MRCoNS) is an important role of clinical microbiology laboratories to avoid treatment failure. The aim of this study was to compare conventional phenotypic methods for detection of methicillin resistance in CoNS isolates. Methicillin resistance was studied among clinical isolates of CoNS in Milad hospital of Tehran. Five different phenotypic methods including E-test MIC, oxacillin screening agar, oxacillin disk diffusion, cefoxitin disk diffusion and CHROMagar MRSA methods were used for detection of methicillin resistant CoNS. During our study, between January and December 2010 in total 114 CoNS strains were isolated from clinical specimens. The majority isolates were from blood culture. Oxacillin and cefoxtin both showed 98 methicillin resistant CoNS. The sensitivity and specificity for these two methods was 95.37% and 100% respectively. The sensitivity and specificity of oxacillin screening agar was similar to those disk diffusion methods. CHROMagar MRSA had low sensitivity in comparison to other methods All isolates including methicillin resistant CoNS were susceptible to vancomycin. Nearly 96% isolates were resistant to penicillin Rate of resistance to oxacillin was 90.35% using E-test MIC method. Our study revealed that all phenotypic methods have high sensitivity and specificity for detection of methicillin resistant CoNS. However the cefoxitin disk test is easy to perform and rapidly becoming the preferred method for detection of oxacillin heteroresistance in CoNS.