Local verification of ceftriaxone gradient strip MIC testing for Neisseria gonorrhoeae after CLSI 2026 breakpoint...

CLSI M100, 36th edition (2026), lowered the ceftriaxone susceptible MIC breakpoint for Neisseria gonorrhoeae from ≤0.25 mg/L to ≤0.12 mg/L, classified 0.25 mg/L as intermediate, and removed ceftriaxone disk diffusion interpretive criteria. We conducted a single-center, retrospective method-comparison study of 111 consecutive, nonduplicate, culture-confirmed isolates with paired ceftriaxone gradient strip MICs, agar dilution MICs, and archived 30-µg disk diffusion zone diameters. Gradient strip readings were prespecified to be rounded upward to the next nominal twofold dilution for comparison with agar dilution. Standardized essential agreement within ±1 dilution was 111/111 (100.0%); because upward rounding can increase apparent essential agreement, essential agreement based on the original continuous readings was also assessed and was 102/111 (91.9%). Using a binary interpretation based on the revised susceptible breakpoint (≤0.125 mg/L, displayed by CLSI as ≤0.12 mg/L, vs >0.125 mg/L), gradient strip and agar dilution classifications agreed for all 111 isolates (100.0%). Complete CLSI 2026 S/I/R categorical agreement was 105/111 (94.6%). The six discrepancies were all minor errors in which gradient strip testing classified an isolate as intermediate, whereas agar dilution classified it as resistant; no major or very major errors occurred. Gradient strip testing identified 12 of 18 isolates classified as resistant by agar dilution, corresponding to 66.7% sensitivity for the resistant category. Both methods classified 28 of 111 isolates (25.2%) as having MICs above the revised susceptible breakpoint; this proportion represents a classification frequency in a method-comparison data set, not a regional prevalence estimate.

Clinical laboratories need practical approaches when antimicrobial susceptibility breakpoints change. For Neisseria gonorrhoeae, the 2026 CLSI revision lowered the ceftriaxone susceptible MIC breakpoint and removed ceftriaxone disk diffusion interpretive criteria. Using paired isolate-level results, this study evaluated a product-specific ceftriaxone gradient strip workflow against agar dilution and examined archived disk diffusion results using the former 2025 CLSI susceptible-zone threshold of 35 mm. After prespecified upward rounding to the reference twofold scale, standardized essential agreement was 100.0% and complete CLSI 2026 S/I/R categorical agreement was 94.6%; all six categorical discrepancies involved gradient strip intermediate results and agar dilution resistant results. The former disk diffusion threshold showed limited specificity in this historical exploratory analysis. These data provide a single-center assessment to inform local verification of gradient strip MIC testing and transition toward MIC-based ceftriaxone surveillance.