ISO/NP TS 20776-3

ISO/TS 20776-3, Clinical laboratory testing and in vitro diagnostic test systems — Susceptibility testing of infectious agents and evaluation of performance of antimicrobial susceptibility test devices — — Part 3: Part 3: Quality performance of disc-diffusion agar method for testing the in vitro activity of antimicrobial agents against rapidly growing aerobic bacteria involved in infectious diseases

Scope

This part of ISO 20776 describes aspects of quality performance for one method, disc diffusion agar, for qualitative determination of general antimicrobial susceptibility or resistance. The disc diffusion interpretive result can be a guide for the clinician, and reflects the activity of the drug under the described test conditions, by taking into account other factors, such as drug pharmacology, pharmacokinetics, or bacterial resistance mechanisms. The disc diffusion test result requires comparison with broth dilution minimal inhibitory concentration test results. The zone diameter of inhibition (in mm) around the disc reflects the general qualitative susceptibility of the micro-organism/antimicrobial agent and can be compared to MICs regarding presumptive susceptibility or resistance of the micro-organism to the antimicrobial agent. In addition, disc diffusion testing distributions can be used to define wild type or non-wild type bacterial populations. Clinical interpretation of the disc diffusion zone of inhibition diameter value is beyond the scope of this part of ISO 20776. Quality performance of the test method is described herein, and modifications of the basic method are required for certain antimicrobial agent - bacteria combinations to facilitate in vitro interpretation. These modifications are included in a separate Annex of this document. It is necessary to compare other susceptibility testing methods (e.g. broth micro-dilution or diagnostic test devices) with the agar disc diffusion method for validation, in order to ensure comparable and reliable results. That is beyond the scope of this document.