Abstract
ABSTRACT
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.
IMPORTANCE
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.