Determination of methicillin resistant Staphylococcus aureus (MRSA) using several phenotypic methods -a report from a tertiary care center at New Delhi, India

Staphylococcus aureus is a serious current health care concern. Rapid, precise identification of MRSA is a prerequisite tool for control of hospital infection. The present study aimed at studying the antibiotic susceptibility and demographic profile of MRSA isolated from clinical specimens and also to assess the reliability of five methods of identifying methicillin resistance. A total of 112 isolates of Staphylococcus aureus isolated from critical areas of the hospital were randomly selected and subjected to various phenotypic methods for determining methicillin resistance. Determination of minimum inhibitory concentration (MIC) for oxacillin by microbroth dilution method was the 'gold standard'. The present study also focuses on the comparison of oxacillin disc diffusion and cef oxitin disc diffusion screening for determination of MRSA. The sensitivity and specificity values for latex agglutination, oxacillin salt agar screening, E-Strip, cefoxitin disc diffusion and oxacillin disc diffusion method are 98.7% and 72.5%, 94.4% and 92.5%, 93.1% and 65.0%, 86.1% and 65%, 83.3% and 90.0 % respectively. No single phenotypic test is completely reliable for the detection of oxacillin resistance in S.aureus. Oxacillin salt agar screening at 6ug/ml and PBP2' detection by latex agglutination method were the most sensitive and specific method for detecting MRSA. Amidst the disc screening methods which are most often used in the smaller laboratory set-up where agar screen may not be functionally feasible for various reasons (economic and performance) cefoxitin disc screening has good sensitivity and specificity overall and perhaps is closest to being the cheap and reliable alternative in these settings.