Laboratory diagnosis of methicillin resistance in Staphylococcus aureus: Genotypic or phenotypic methods?

Sir,

Bhutia et al. add information to the clinical microbiology literature with one more study addressing the accuracy of different laboratory methods for the detection of methicillin resistance in Staphylococcus aureus,[1] reporting, as other authors, limitations in accuracy of the phenotypic methods.

We have also previously published on this subject, reporting very good accuracies for Etest and oxacillin agar screening plate, but relatively low accuracies for oxacillin and cefoxitin disk diffusion.[23] Although mecA polymerase chain reaction seems to be the most accurate method, it is still not available throughout the world due to financial and technical issues. In addition, it does not detect the other rare but possible mechanisms of oxacillin resistance, including other modified penicillin-binding proteins and beta-lactamase overproduction.[4]

As we have previously proposed,[23] a more appropriate option for increasing the sensitivity of methicillin resistance detection would be the concomitant use of two phenotypic methods, such as oxacillin and cefoxitin disk diffusion, or oxacillin agar screening plate and cefoxitin disk diffusion. Any isolate resistant by at least one of the tests should be reported as resistant. In our opinion, this combination approach would, with low cost, increase the sensitivity without decreasing the specificity significantly.