Etest® versus broth microdilution for ceftaroline MIC determination with Staphylococcus aureus: results from PREMIUM, a European multicentre study.
J Antimicrob Chemother. 2016 Oct 20.
Cantón R1, Livermore DM2, Morosini MI3, Díaz-Regañón J4, Rossolini GM5,6; PREMIUM Study Group.
1Servicio de Microbiología, Hospital Universitario Ramón y Cajal and Instituto Ramón y Cajal de Investigación Sanitaria (IRYCIS), Madrid, Spain firstname.lastname@example.org
2Norwich Medical School, University of East Anglia, Norwich, UK.
3Servicio de Microbiología, Hospital Universitario Ramón y Cajal and Instituto Ramón y Cajal de Investigación Sanitaria (IRYCIS), Madrid, Spain.
4AstraZeneca Medical Department, Madrid, Spain.
5Department of Experimental and Clinical Medicine, University of Florence and Clinical Microbiology and Virology Unit, Florence Careggi University Hospital, Florence, Italy.
6Department of Medical Biotechnologies, University of Siena, Siena, Italy.
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.
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.
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.
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.