Evaluation of Five Phenotypic Methods for Detection of Methicillin Resistant Staphylococcus aureus (MRSA)

Rapid and accurate detection of methicillin resistant Staphylococcus aureus (MRSA) is an important role of clinical microbiology laboratories to avoid treatment failure. The aim of this study was to compare conventional methods against the E-test minimum inhibitory concentration (MIC) method to determine the best phenotypic method. Methicillin resistance was studied among clinical isolates of S. aureus from April to October 2009 in Milad Hospital of Tehran. These methods included E-test MIC, oxacillin screen agar, oacillin disk diffusion, cefoxitin disk diffusion, and CHROMagar- MRSA methods. Out of 294 isolates of S. aureus, one hundred and six (36%) strains of MRSA were isolated from clinical specimen. Oxacillin screen agar and CHROMagar-MRSA showed both 110 MRSA isolates. The sensitivity and specificity for these two methods were 100% and 97.9%, respectively. The sensitivity and specificity of oxacillin disk diffusion method was similar to those of oxacillin screen and CHROMagar-MRSA. One hundred and eight strains of S. aureus were MRSA by cefoxitin disk diffusion method. The sensitivity and specificity of cefoxitin disk diffusion method was 100% and 98.1% respectively. All isolates including MRSA were susceptible to vancomycin. Nearly al MRSA isolates were resistant to erythromycin, clindamycin, chloramphenicol, tetracycline, ceftriaxone and ciprofloxacin. All phenotypic methods had high sensitivity and specificity for detection of MRSA. However, cefoxtin disk diffusion method in comparison to other methods had higher specificity.