ISO 20776-1:2019

Overview

ISO 20776-1:2019 specifies the broth micro-dilution reference method for determining minimum inhibitory concentrations (MICs) of antimicrobial agents against rapidly growing aerobic bacteria involved in infectious diseases. As a reference method for antimicrobial susceptibility testing (AST), it standardizes procedures-media, antimicrobial preparation, inoculum, incubation and result reading-so MICs are reproducible and comparable across laboratories. The method uses micro-dilution trays (final well volume ≤200 µl) and is intended for pure cultures that grow well in standardized Mueller‑Hinton broth.

Key technical topics and requirements

• Test format: Broth micro-dilution in polystyrene micro-trays; typical well volumes indicated (e.g., 50 µl working solution + 50 µl inoculum or 100 µl working solution with small inoculum volume).
• Medium: Standardized Mueller‑Hinton broth (requirements and special situations addressed in Annex A and D).
• Antimicrobial agents: Use of certified powders from manufacturers (potency, lot number, expiry); preparation of stock and working solutions, appropriate solvents/diluents (Annex B and C).
• Inoculum preparation: Defined methods (broth culture or direct colony suspension) to achieve standardized colony-forming units per ml (CFU/ml).
• Incubation and reading: Defined incubation conditions and procedures for reading visible growth to determine the MIC; guidance on special test situations where MIC may be unreliable.
• Quality control: Use of reference strains and QC practices to ensure intra- and inter-laboratory reproducibility (details provided in the main text and annexes).
• Scope limits: The document is a laboratory reference method; clinical interpretation (breakpoint setting) is beyond its scope though MICs enable categorization (S, I, R) when interpreted with breakpoints from bodies such as EUCAST or CLSI.

Practical applications and typical users

• Clinical microbiology laboratories performing quantitative AST for patient management when MICs are required.
• Reference and public-health laboratories conducting surveillance of antimicrobial resistance and defining wild-type MIC distributions.
• Manufacturers and evaluators of diagnostic AST devices (disc diffusion, automated systems) who must validate new or alternative methods against this reference broth micro-dilution method.
• Research groups comparing new antimicrobial agents or studying resistance mechanisms.

Related standards and notes

• Comparable methods and interpretive criteria are published by CLSI and EUCAST; ISO 20776-1 aligns methodologically with these organizations.
• Part of ISO/TC 212 (Clinical laboratory testing and in vitro diagnostic test systems) and complements other parts of the ISO 20776 series.
• Keywords: ISO 20776-1:2019, broth micro-dilution, MIC, antimicrobial susceptibility testing, Mueller‑Hinton broth, AST validation, clinical microbiology, quality control.