LAUSR

Objectives To compare the concordance of ceftaroline MIC values by reference broth microdilution (BMD) and Etest (bioMérieux, France) for MSSA and MRSA isolates obtained from PREMIUM (D372SL00001), a European multicentre study. Methods Ceftaroline MICs were determined by reference BMD and by Etest for 1242 MSSA and MRSA isolates collected between February and May 2012 from adult patients with community-acquired pneumonia or complicated skin and soft tissue infections; tests were performed across six European laboratories. Selected isolates with ceftaroline resistance in broth (MIC >1 mg/L) were retested in three central laboratories to confirm their behaviour. Results Overall concordance between BMD and Etest was good, with >97% essential agreement and >95% categorical agreement. Nevertheless, 12 of the 26 MRSA isolates found resistant by BMD scored as susceptible by Etest, with MICs ⩽1 mg/L, thus counting as very major errors, whereas only 5 of 380 MRSA isolates found ceftaroline susceptible in BMD were miscategorized as resistant by Etest. Twenty-one of the 26 isolates with MICs of 2 mg/L by BMD were then retested twice by each of three central laboratories: BMD MICs of 2 mg/L were consistently found for 19 of the 21 isolates. Among 147 Etest results for these 21 isolates (original plus six repeats per isolate) 112 were >1 mg/L. Conclusions BMD and Etest have good overall agreement for ceftaroline against Staphylococcus aureus; nevertheless, reliable Etest-based discrimination of the minority of ceftaroline-resistant (MIC 2 mg/L) MRSA is extremely challenging, requiring careful reading of strips, ideally with duplicate testing.