Practical use of piperacillin MICs to distinguish β-lactamase-producing isolates from non-producing isolates in Haemophilus influenzae

Aim Chromogenic cephalosporins are widely used to detect β-lactamase production in Haemophilus influenzae. However, an alternative method is required in the case of reagent supply interruption. The aim of this study was to evaluate the diagnostic performance of piperacillin (PIPC) MICs for distinguishing β-lactamase-producing isolates from non-producing isolates in H. influenzae.Materials and methods The MICs of PIPC were measured for 132 H. influenzae isolates with identified genetic mechanisms of resistance. Diagnostic performance of PIPC was evaluated.Results The MICs of PIPC increased exclusively in β-lactamase-producing isolates, ranging from 4 µg/mL to >64 µg/mL. When a threshold of 4 µg/mL was used to distinguish β-lactamase-producing isolates from non-producing isolates, the sensitivity and specificity were 100%.Conclusions PIPC MICs may be useful supplementary indicator to distinguish β-lactamase producing H. influenzae from non-producing isolates, particularly when chromogenic cephalosporin-based testing is unavailable because of supply interruptions.