Andrew Henderson, Matthew P Cheng, Ka Lip Chew, Geoffrey W Coombs, Joshua S Davis, Jennifer M Grant, Dan Gregson, Stefano G Giulieri, Benjamin P Howden, Todd C Lee, Vi Nguyen, Jocelyn M Mora, Susan C Morpeth, James O Robinson, Steven Y C Tong, Sebastiaan J Van Hal, Reem Abdul-Hameed, Michael Addidle, Eugene Athan, Max Bloomfield, Katherine Bond, Carly Botheras, Susan Bradbury, Alex Carignan, Wilson Chan, Rose Contronei, Louise Cooley, Julie Creighton, Peter Daley, Nick Daneman, Matthew Diggle, Dragana Drinković, Juliet Elvy, Nadine Flett, Hong Foo, Jaimie Frazer, Nesrin Ghanem-Zoubi, Anna Goodman, Clair Gregory, Jock Harkness, Melissa Hoddle, Julia Howard, Ali Jissam, Kristin Kalan, Pankaja Kalukottege, Peter Kelley, Tony M Korman, Robert Kozak, Philippe Lagace-Wiens, Adriana Larrotta, Queenie Leong, Marcel Leroi, David Lorenz, Philippe Martin, Susy Mathew, Belinda McEwan, Andrew McGlinchey, Genevieve McKew, Brendan McMullan, John Merlino, Angela Mrkusich, Sean Munroe, Peter Newton, Leighanne O Parkes, Dina Pollak, Murray Robinson, Sharon Roessler, Madeline Russell, Maria Satie, Narinder Sharma, Mendelsohn Sigal, Richard Streitberg, Vincent Tan, Koen van der Werff, Evangeline S Virey, Heather Wilson, Deb Yamarura, Yang Yu, Helen Ziochos

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. 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. 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. 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.