Comparison of the performance of Disk diffusion, Agar screening and E-test methods with Real-time PCR for the detection of methicillin resistant coagulase negative Staphylococcus strains isolated from clinical samples of Shahrekord university hospitals, 2008

Coagulase Negative staphylococci (CoNS) are nosocomial pathogens. The main objective of the study was to compare the performance of disk diffusion, E-test and agar screening methods with Real-time PCR technique for detection of methicillin resistance in coagulase negative staphylococci (MRCoNS), using Taqman® Real-time PCR for mecA as the “gold standard” comparison assay.

88 coagulase negative Staphylococcus isolates were identified out of 284 Staphylococcus isolates collected from Hajar and Kashani hospitals-shahrekod, Iran. Methicillin resistance strains were identified by several

Disk diffusion, Agar screening, E-test and Real-time PCR. The results of the tested methods were compared with those of the Real-time PCR by Chi square or Fisher’s exact tests.

Of the 88 coagulase negative Staphylococcus isolates tested, 46 isolates (52.3%) were mecA-positive and 42 isolates (47.7%) were mecA-negative. The results of all the tested methods had a statistically significant agreement with those of Real-time PCR. The E-test was 100% sensitive and specific for mecA presence. The sensitivity and specificity of oxacillin agar screen (6.0 μg/ml) method were 96% and 98%, respectively and the sensitivity and specificity of oxacillin Disk diffusion method were 91% and 90%, respectively.

In the present study, E-test is proposed as the best phenotypic method. In case economic issue matters, the oxacillin agar screening method (6.0 μg/ml), which is suitable for the detection of MRCoNS due to its accuracy and low cost, is recommended.