Detection of Low-Level Methicillin-Resistant Staphylococcus aureus with Commercially Available Tests

We were delighted to see the recent article by Felten et al. on the detection of low-level methicillin-resistant Staphylococcus aureus (MRSA) (1). The study is, to our knowledge, the first to specifically test the MRSA-screen test on low-level MRSA strains. However, Felten et al. concluded that the MRSA-screen test is the best commercially available test for distinguishing very-low-level class 1 MRSA from methicillin-susceptible S. aureus. We would therefore like to point the authors' attention to the EVIGENE MRSA Detection Kit (Statens Serum Institut, Copenhagen, Denmark), which is also a commercially available product for detecting methicillin resistance in staphylococci. This kit is a DNA probe-based hybridization assay for detecting the mecA gene in staphylococci. Each test also detects the nuc gene for differentiating between S. aureus and coagulase-negative staphylococci as well as internal positive and negative controls. The kit uses no amplification steps; thus, the issues of contamination applicable to PCR do not apply to this kit. The kit result, which is obtained in 3.5 h, is visually read as yellow or blank. We have evaluated the EVIGENE MRSA Detection Kit with 275 S. aureus isolates, including 18 borderline-resistant strains; the results showed both the sensitivity and the specificity to be 100% for the entire collection (3). For coagulase-negative staphylococci, 242 strains have been evaluated (128 mecA-positive and 114-mecA negative strains). Of these 242 strains, 5 (2%) produced a positive control signal below the cutoff defined for the kit, thus rendering the five tests invalid. For the remaining 239 strains, the sensitivity and specificity were as for S. aureus: both 100% (2).