Performance of Phenotypic Tests for Detection of Penicillinase in Staphylococcus aureus Isolates from Australia

Recent studies have shown that chromogenic cephalosporin tests are inferior to disc zone edge tests in detecting penicillinase in Staphylococcus aureus isolates, resulting in a change to CLSI and EUCAST guidelines in 2012. We sought to confirm these findings using Australian isolates and compare the performance of the CLSI and EUCAST methods, which use different disc strengths, penicillin at 10 units (P10) and 1 unit (P1), respectively. Using blaZ PCR as the reference standard, the sensitivities of the tests for detection of penicillinase production were as follows: Cefinase disc test, 24/38 isolates (63%); P10 disc zone edge test, 34/38 isolates (89%); P10 disc diameter test, 25/38 isolates (66%); P1 disc zone edge test, 38/38 isolates (100%); and P1 disc diameter test, 38/38 isolates (100%). We also found that the P10 disc zone edge test reading was interpreted differently by the clinical laboratory and the study investigators in 11% of instances. Our findings support those of previous studies showing that chromogenic cephalosporin-based beta-lactamase tests are inferior to disc methods in detecting S. aureus penicillinase. We also conclude that the EUCAST method using the P1 disc has the best performance, particularly because the P1 disc zone diameter reading closely correlated with penicillinase production and reading of the disc zone diameter is less subjective than reading of the zone edge.