Rapid detection of mecA-positive and mecA-negative coagulase-negative staphylococci by an anti-penicillin binding protein 2a slide latex agglutination test

A rapid slide latex agglutination (LA) test, MRSA-Screen (Denka Seiken Co., Niigata, Japan), which detects PBP 2a, was tested for its ability to diffe rentiate between mecA-positive and -negative coagulase-negative staphylococ ci. A total of 463 isolates from 13 species were included in the study. The mecA gene was detected by PCR, and the oxacillin MIC was determined by the agar dilution method according to the guidelines of the National Committee for Clinical Laboratory Standards (NCCLS), The LA test was performed with oxacillin-induced isolates, The true-positive and true-negative results wer e defined on the basis of the presence or the absence of the mecA gene. By PCR, 251 isolates were mecA positive and 212 were mecA negative. The sensit ivities, specificities, and positive and negative predictive values for the LA test compared to the NCCLS breakpoint for oxacillin resistance (greater than or equal to 0.5 mg/liter) were as follows: for the LA test, 100, 99.5 , 99.6, and 100%, respectively; for the NCCLS breakpoint, 100, 60.8, 75.1, and 100%, respectively. One hundred twenty-five mecA-positive isolates were also tested by the LA test without induction of PBP 2a; only 72 (57.6%) ga ve a positive result and required 3 to 15 min for reaction. With induction, all 251 isolates were positive within 3 min. The LA test was reliable in c lassifying mecA-negative isolates, but it classified isolates for which the oxacillin MIC was greater than or equal to 0.5 mg/liter as oxacillin susce ptible. For the reliable detection of oxacillin resistance by the MRSA-Scre en in coagulase-negative staphylococci, induction of the mecA gene appears to be necessary.