Clinical and Laboratory Standards Institute standard M27—Reference Method for Broth Dilution Antifungal Susceptibility Testing of Yeasts describes a method for testing the susceptibility to antifungal agents of yeasts that cause invasive fungal infections, including Candida spp. and Cryptococcus neoformans. Selection and preparation of antifungal agents, implementation and interpretation of test procedures, and the purpose and implementation of QC procedures are discussed. A careful examination of the responsibilities of the manufacturer and the user in QC is also presented.

This standard replaces the previous edition of the approved standard, M27-A3, published in 2008. Several changes were made in this edition, including:

• General: – Revised document format and organization to reflect the CLSI quality system essential and path of workflow document templates and the updated CLSI style – Updated references to the previous informational supplements (M27-S4 and M44-S3) to reflect CLSI document M60,12 the new supplement for broth dilution and disk diffusion yeast susceptibility testing – Added references to epidemiological cutoff values and CLSI documents M5713 and M5914

• Subchapter 1.4.2, Definitions: – Revised the breakpoint and interpretive category definitions for consistency with other CLSI antimicrobial susceptibility testing documents – Added definitions for “wild-type” and “non-wild-type” – Deleted all uses of the phrase “interpretive criteria”

• Chapter 3, Antifungal Susceptibility Testing Process: – Added an antifungal susceptibility testing process flow chart – Replaced procedural text with step-action tables – Added an explanation for deleting breakpoints for itraconazole and flucytosine – Changed recommended reading time for broth microdilution to 24 hours only for clinical isolates and QC strains (24 and/or 48 hours was accepted for some antifungal agents in M27-A3) – Deleted results interpretation information for ketoconazole

This standard describes a reference method for testing susceptibility to antifungal agents of yeasts that cause infections, including Candida spp. and Cryptococcus spp. The intended users are laboratory personnel who perform antifungal susceptibility testing on yeasts. The focus is on developing relevant breakpoints for available antifungal agents. 10 and reference minimal inhibitory concentration (MIC) ranges for broth dilution testing of both established and newly introduced antifungal agents.11 For MIC breakpoints, interpretive categories, and MIC ranges for quality control (QC) isolates, refer to CLSI document M60.12 This method has not been extensively validated for the yeast forms of dimorphic fungi, such as Blastomyces dermatitidis or Histoplasma capsulatum. Also, testing filamentous fungi (moulds) introduces several additional standardization problems not covered by this procedure and is not included. For an antifungal broth dilution susceptibility testing reference method for filamentous fungi, refer to CLSI document M38.15 Commercially available susceptibility test systems are out of scope for this standard. It is recommended that users of these systems refer to the manufacturer’s instructions as outlined in the package insert.