Implementation and evaluation of gradient strip antimicrobial susceptibility testing in US public health laboratories to respond to resistant gonorrhea.

BACKGROUND Gradient strip antimicrobial susceptibility testing (AST) using Etest® is conducted by local public health jurisdictions participating in the Strengthening the U.S. Response to Resistant Gonorrhea (SURRG) program to inform public health responses to resistant gonorrhea. Proficiency testing results across the participating laboratories were analyzed and a comparison of Etest® with the agar dilution method was conducted. METHODS Laboratories participating in SURRG performed Etest® for azithromycin (AZM), cefixime (CFX), and ceftriaxone (CRO). Concurrence between minimum inhibitory concentrations (MICs) obtained with Etest® versus the agar dilution method using corresponding isolates was defined as +/- 1 double dilution. Specific levels of reduced susceptibility were termed "alerts" and included isolates with the following MICs: ≥ 2.0 μg/ml (AZM), ≥ 0.25 μg/ml (CFX), and ≥ 0.125 μg/ml (CRO). Categorical (alert/non-alert) agreement was calculated for MICs determined using Etest® and agar dilution methods. RESULTS SURRG laboratories had high proficiency testing scores (≥98%) and low levels of inter-laboratory variations in MICs. The overall concurrence of MICs (essential agreement) determined using agar dilution and Etest® was 96% (CRO), 96% (CFX), and 95% (AZM). Depending on the antibiotic tested, between 27-66% of isolates with alert MICs determined by Etest® also had alert MICs using the reference agar dilution methodology, however most of these alert MICs were detected at threshold levels. CONCLUSIONS This study demonstrates that MICs produced by SURRG laboratories using Etest® have a high level of concurrence with agar dilution. Although confirmation of specific alert MICs varied, Etest® facilities rapid detection and response to emerging resistant gonorrhea.