The Clinical Impact of a Negative Molecular β-Lactamase Gene Test for Enterobacteriaceae : Let’s Not Let Perfect Be the Enemy of Really Good

Abstract

We read with interest “Evaluation of Empiric β-Lactam Susceptibility Prediction among Enterobacteriaceae by Molecular β-Lactamase Gene Testing” by Spafford and colleagues (1). We commend the authors for their analysis of nationwide susceptibility/resistance gene data to provide an examination beyond that of an individual institution. In their analysis of 5,739 Enterobacteriaceae from 72 hospitals in the United States, including 683 (11.9%) ceftriaxone-resistant isolates, the authors demonstrated that using the absence of a resistance marker detected by Verigene (CTX-M, KPC, or NDM) to predict ceftriaxone susceptibility would result in a very major error (VME) rate of 18.6%. That is, 127/683 isolates that were resistant to ceftriaxone would not be detected by the molecular test. The authors then assess the impact that this VME rate would have at different resistance rates and alarmingly demonstrate that if the rate of ceftriaxone resistance at one’s institution was 50%, molecular testing would fail to capture resistance in 1 out of 10 isolates tested! This leads the authors to caution against using genotypic results for de-escalation efforts and stress the benefit of rapid phenotypic tests.