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CLSI Publishes Second Edition of M24S—Performance Standards for Susceptibility Testing of Mycobacteria, Nocardia spp., and Other Aerobic Actinomycetes


Malvern, Pennsylvania, USA—The Clinical and Laboratory Standards Institute (CLSI) has published M24S—Performance Standards for Susceptibility Testing of Mycobacteria, Nocardia spp., and Other Aerobic Actinomycetes, 2nd Edition. M24S includes the minimal inhibitory concentrations and QC ranges developed following the standards described in CLSI document M24, which includes susceptibility testing procedures for Mycobacterium tuberculosis complex (MTBC), clinically significant slowly and rapidly growing mycobacterial species, Nocardia spp., and other aerobic actinomycetes. Also included in this standard are recommendations for selecting agents for first-line and second-line drug testing, organism group–specific methodologies, reporting recommendations, and organism quality control criteria.

The M24 standard also includes recommendations for using commercial broth susceptibility methods with shorter incubation times and information on molecular methods for detecting drug resistance and their integration with culture-based methods.

Nancy L. Wengenack, PhD, D(ABMM), Co-Chairholder of M24S said, “The M24S document provides updated breakpoints, QC organisms, and QC ranges for M. tuberculosis complex, nontuberculous mycobacteria and aerobic actinomycete susceptibility testing. For example, after review of available data, the critical concentration for rifampin testing with M. tuberculosis complex was adjusted to match the current World Health Organization standard concentration which brings both the CLSI and WHO testing recommendations into alignment.”

CLSI will be co-hosting a webinar with College of American Pathologists—What’s New in Susceptibility Testing of Mycobacteria—based on M24S. The live webinar will be held on

Thursday, May 4 from 2:00-3:00 PM Eastern (US) Time.

Speakers will:

  • Discuss current methods for phenotypic and genotypic antimicrobial susceptibility testing (AST) of M. tuberculosis complex (MTBC).
  • Review new and evolving standards for non-tuberculous Mycobacterium spp.
  • Evaluate updated guidance from the recently published CLSI M24S.
  • Review recent changes that impact AST for MTBC.

For more information on M24S, contact Joanne Christopher at jchristopher@clsi.org or +1.484.588.5907.



CLSI is the leading global non-profit laboratory medicine standards development organization, with over 24,000 individuals with membership access, 2,000 volunteers, and 250 products. CLSI standards are recognized by laboratories, accreditors, and government agencies worldwide as the best way to improve medical laboratory testing. Organizations use CLSI standards to improve their testing outcomes, maintain accreditation, bring products to market faster, and navigate regulatory hurdles.


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