To be or not to be? (Resistant)

Abstract

Background Our laboratory changed routine antibiotic susceptibility testing from the Clinical and Laboratory Standards Institute (CLSI) to the European Committee on Antimicrobial Susceptibility Testing (EUCAST) method in August 2014. We have since observed a sudden increase in amoxicillin/clavulanate (Augmentin) resistant, beta-lactamase positive, Haemophilus influenzae . Aim This Quality Assurance Project compares H. influenzae susceptibility using breakpoints from EUCAST and CLSI methodology. The null hypothesis: EUCAST method is more sensitive, therefore increased detection of Augmentin resistant strains of H. influenzae is expected. Aim – to determine a gold standard for H. influenzae susceptibility testing. Method H. influenzae isolates were predominantly from respiratory specimens. Disc susceptibility and Augmentin E-tests were performed on every isolate using EUCAST and CLSI methods. Results The CLSI method provided consistent results; disc susceptibility correlated well with E-test results. The EUCAST results were variable; E-test results often did not correlate with disc susceptibility data. Discussion Using the EUCAST method, we identified more H. influenzae that were Augmentin resistant, beta-lactamase producers. Using the CLSI we would have mis-reported these isolates as susceptible to Augmentin. This study raises two important questions: which of the two susceptibility testing methods provides the accurate in vitro results, likely to correlate with clinical effectiveness; and, which method should be recommended for routine use in a diagnostic laboratory?