Comparing BSAC's oxacillin 1µg disc diffusion method with the cefoxitin 10µg disc diffusion method in the identification of methicillin resistance in Staphylococcus aureus

Abstract

Staphylococcus aureus is a significant cause of soft tissue infection. The emergence of methicillin resistant S. aureus has resulted in increasing treatment difficulty. With break-out epidemics of community acquired S. aureus becoming more frequent, the pressure on public health laboratories to provide accurate identification of resistant strains is becoming increasingly more important. Using recommendations from The British Society of Antimicrobial Chemotherapy, oxacillin and cefoxitin disc diffusion methods were compared at 20 and 44hrs incubation with PCR detection of the mec A gene to determine which method was proved to be the better indicator of methicillin resistance. In this investigation it was found that using the Cefoxitin disc diffusion method at 20hrs incubation provided the highest overall accuracy (96.5%), sensitivity (99.0%) and negative predictive value (99.0%). 20hrs incubation using the oxacillin disc diffusion method was less accurate (95.5%), sensitive (94.9%) and had a lower negative predictive value (95.2%), but provided a higher specificity (96.1%) and positive predictive value (95.9%). Increasing incubation times for both techniques increased sensitivity and negative predictive value to 100% but resulted in a sharp deterioration of specificity, positive predictive value and overall accuracy. Overall using the data acquired from this investigation, the cefoxitin disc diffusion method is suggested as a suitable method for the routine clinical identification of methicillin resistant S. aureus.