Annals of laboratory medicine
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Guidelines recommend that clinical laboratories perform phenotypic tests (nitrocefin-based test and penicillin 10-U [P10] or 1-U [P1] zone edge tests) to detect penicillinase in Staphylococcus aureus isolates. This study aimed to assess the prevalence of blaZ encoding penicillinase and perform various phenotypic tests in S. aureus isolates from Japan. ⋯ Using blaZ PCR as a standard, we observed a low sensitivity (50%) and positive predictive value (PPV, 50%) of the nitrocefin-based test, low PPV (18.2%) of the P10 zone edge test, low sensitivity (50%) of the P10 diffusion test, low PPV (50% and 22.2%) of the P2 zone edge test and P2 diffusion test, respectively, and low sensitivity (50%) of the cloverleaf test. These data suggest a low performance (sensitivity and PPV) of these six phenotypic tests because of the low prevalence (1%) of blaZ in S. aureus isolates from Japan.