Diagnostic Discordance in ESBL Detection: HiCrome™ Chromogenic Agar and DDST
DOI:
https://doi.org/10.64149/Keywords:
ESBL,Enterobacterales, Chromogenic agar, Double disc synergy test, Diagnostic agreement, Cohen’s kappa,McNemar’s test, India.Abstract
Background: Extendedspectrum βlactamases (ESBLs) compromise the efficacy of thirdgeneration cephalosporins and are a major driver of antimicrobial resistance among Enterobacterales. Rapid phenotypic screening (chromogenic agar) and confirmatory phenotypic tests (doubledisc synergy test, DDST) are widely used in routine laboratories, but discordance between these methods has diagnostic and therapeutic implications.
Objectives: To evaluate the prevalence of phenotypic ESBL indicators among thirdgeneration cephalosporinresistant Enterobacterales and to quantify agreement and discordance between HiCrome™ ESBL chromogenic agar screening and DDST confirmation in a tertiarycare centre in Haryana.
Methods: Observational study of 150 Enterobacteralesisolates procured at Microbiology laboratory that were resistant to ceftazidime, cefotaxime or ceftriaxone by CLSI disk diffusion. Chromogenic screening (HiCrome™ ESBL Agar) and DDST were performed on all 150 isolates. Agreement was assessed using Cohen’s κ and McNemar’s test; stratified analyses and logistic regression explored predictors of discordance. PCR confirmation was planned for dualpositive isolates in a subsequent study.
Results: Of 150 isolates, 119 (79.3%) were chromogenicpositive and 73 (48.7%) were DDSTpositive. The 2×2 contingency yielded: both positive = 73; chromogenic+/DDST− = 46; chromogenic−/DDST+ = 0; both negative = 31. Observed agreement = 0.693; expected agreement = 0.491; Cohen’s κ = 0.398 (moderate agreement). McNemar’s test (continuity corrected) χ² ≈ 44.02, p < 0.0001, indicating significant directional discordance (predominantly chromogenic positives lacking DDST confirmation). Discordance was more frequent among Escherichia coli and urine isolates.
Conclusions: HiCrome™ chromogenic screening yields a high proportion of presumptive ESBL positives but shows only moderate agreement with DDST; many chromogenic positives lack clavulanate synergy. The observed discordance has important implications for antimicrobial stewardship and infection control.
