Beyond Categorical Agreement: Embracing Essential Agreement for Robust AST Proficiency Testing in CLSI-Dominant India

The updated ISO 20776-2 (edition 2, published in 2021) removed categorical agreement (CA) as a core metric for validation of antimicrobial susceptibility testing (AST) devices which are based on MIC determination, prioritising essential agreement (EA) and bias due to CA's dependence on evolving clinical breakpoints. We discuss impacts on external quality assessment (EQA), propose EA/bias as primary metrics, and compare EUCAST vs CLSI CA frameworks for antimicrobial stewardship (AMS). Indian proficiency schemes, reliant on CLSI guidelines, traditionally score participants primarily on CA. This approach challenges longitudinal comparability when CLSI breakpoints change. EUCAST's refined "I as Susceptible, increased exposure" reduces ambiguity but increases very major discrepancies; CLSI's broader "I" accommodates technical error but risks overcalling intermediates. Hybrid approaches are recommended for Indian CLSI based EQA.