Evaluation of the mrsa-screen test in detecting oxacillin resistance in community and hospital isolates of staphylococcus aureus

The MRSA-Screen Test (Denka Seiken Co., Japan), a latex agglutination test to detect penicillin-binding protein 2a, was compared with PCR for the detection of oxacillin resistance in Staphylococcus aureus. A total of 77 oxacillin-sensitive and 269 oxacillin-resistant (ORSA) isolates were evaluated. Of the ORSA isolates, 186 were nonmultiresistant (NORSA), defined as being resistant to two or fewer antibiotics other than β-lactams. Eighty-three were multiresistant ORSA (MORSA) strains. If PCR is considered the gold standard test, then the sensitivity, specificity, positive and negative predictive values of the MRSA-Screen Test were 100, 99, 99 and 100%, respectively. The endpoint was hard to read with NORSA strains that took longer than 60s to react. MORSA strains took a median 12 s (range 5-60s) to give a positive reaction with the MRSA-Screen Test, whereas NORSA strains took a median 30s (range 5-180s), a difference which was significantly different (P<0.0001, two-tailed Mann-Whitney unpaired two sample test). NORSA strains had an MIC₅₀ of 128mg/l and MIC₉₀ of 256mg/l, whereas MORSA strains had an MIC₅₀ and MIC₉₀ of >256mg/l. The time that the MRSA-Screen Test took to agglutinate with ORSA strains correlated weakly with the MIC (r² = 0.26). Detection of methicillin resistance cost AUD$9 per isolate with the MRSA-Screen Test, compared with AUD$13 per isolate with meca PCR. The MRSA-Screen Test gave excellent sensitivity and specificity, and was quicker and cheaper than PCR. The full 3min should be allowed to elapse before calling a test negative. Organisms giving indeterminate reactions should be tested for the mecA gene by PCR.