A multi-site, international laboratory study to assess the performance of penicillin susceptibility testing of Staphylococcus aureus

Abstract

Objectives: There is clinical uncertainty over the optimal treatment for penicillin-susceptible Staphylococcus aureus (PSSA) infections. Furthermore, there is concern that phenotypic penicillin susceptibility testing methods are not reliably able to detect some blaZ-positive S. aureus.

Methods: Nine S. aureus isolates, including six genetically diverse strains harbouring blaZ, were sent in triplicate to 34 participating laboratories from Australia (n = 14), New Zealand (n = 6), Canada (n = 12), Singapore (n = 1) and Israel (n = 1). We used blaZ PCR as the gold standard to assess susceptibility testing performance of CLSI (P10 disc) and EUCAST (P1 disc) methods. Very major errors (VMEs), major error (MEs) and categorical agreement were calculated.

Results: Twenty-two laboratories reported 593 results according to CLSI methodology (P10 disc). Nineteen laboratories reported 513 results according to the EUCAST (P1 disc) method. For CLSI laboratories, the categorical agreement and calculated VME and ME rates were 85% (508/593), 21% (84/396) and 1.5% (3/198), respectively. For EUCAST laboratories, the categorical agreement and calculated VME and ME rates were 93% (475/513), 11% (84/396) and 1% (3/198), respectively. Seven laboratories reported results for both methods, with VME rates of 24% for CLSI and 12% for EUCAST.

Conclusions: The EUCAST method with a P1 disc resulted in a lower VME rate compared with the CLSI methods with a P10 disc. These results should be considered in the context that among collections of PSSA isolates, as determined by automated MIC testing, less than 10% harbour blaZ. Furthermore, the clinical relevance of phenotypically susceptible, but blaZ-positive S. aureus, remains unclear.

Figure 1.

Penicillin zone size (mm) distribution by method. The number of laboratory tests by diameter of zone size split and coloured by the isolates blaZ PCR result (detected—red; not detected—grey). (a) and (b) depict results using the EUCAST (P1 disc) and the CLSI (P10 disc) methods, respectively, with method breakpoints indicated by the dashed lines. This figure appears in colour in the online version of JAC and in black and white in the print version of JAC.

Figure 2.

Overall laboratory penicillin confirmatory testing results showing aggregated zone diameter by isolate. Aggregated penicillin zone diameter (mm) box and whisker plot (median with 95% CI) for the nine isolates in triplicate labelled A to I on the x-axis. Boxes filled in red represent isolates that are blaZ PCR positive. (a) and (b) depict results using the EUCAST (P1 disc) and the CLSI (P10 disc) methods, respectively. The numbers above the triplicate groups indicate the percentages of correct results. This figure appears in colour in the online version of JAC and in black and white in the print version of JAC.