Specimen Requirements

Stability

Days Performed

Sun - Sat

Turnaround Time

11 days

Methodology

Special Info

Yeast susceptibility testing requires a pure, actively growing culture; mixed or non-viable isolates may lead to invalid results. Accurate species-level identification is essential, as clinical breakpoints vary by species. Testing may be delayed or limited if identification is not provided or not ordered concurrently. The standard antifungal panel does not include 5-fluorocytosine —if susceptibility to this agent is clinically necessary, please contact the laboratory. MIC interpretations are based on the most recent CLSI guidelines. For antifungal agents lacking established breakpoints, only the MIC value is reported without categorical interpretation. As always, susceptibility results should be interpreted in the context of the patient’s clinical condition, site of infection, and therapeutic considerations, since MIC values alone may not fully predict treatment outcomes.

Clinical Info

This test determines the antifungal susceptibility of rapidly growing, non-fastidious yeast isolates—including Candida and Cryptococcus—to a comprehensive panel of antifungal agents. It is performed on pure yeast cultures to support clinical decision-making and guide patient management. Susceptibility testing follows Clinical and Laboratory Standards Institute (CLSI) guidelines, using established breakpoints and interpretive criteria. The antifungal agents tested include amphotericin B, anidulafungin, caspofungin, fluconazole, isavuconazole, itraconazole, micafungin, posaconazole, rezafungin, and voriconazole. Note that 5-fluorocytosine is not included in this panel. Fluconazole and Micafungin will be reported routinely, Amphotericin B will be reported dependent on source. Other antifungals will be reported based on resistance seen to routinely reported antifungals or by request. Testing is performed using the TREK Sensititre™ YeastOne™ YO11 colorimetric microdilution broth method. Each susceptibility plate is pre-loaded with standardized concentrations of antifungal agents and a colorimetric growth indicator. After inoculating the plate with a standardized yeast suspension and incubating at 35°C for 24 to 48 hours (up to 72hours for Cryptococcus species), minimum inhibitory concentrations (MICs) are determined. MICs are identified as the lowest concentration of each antifungal agent that prevents visible yeast growth, as indicated by the absence of color change in the test medium. MIC values are interpreted using clinical breakpoints to categorize isolates as susceptible, intermediate, susceptible-dose dependent, or resistant, in accordance with current guidelines from the Clinical and Laboratory Standards Institute (CLSI). When CLSI has not established clinical breakpoints for a specific antifungal agent, only the MIC value is reported without an interpretive category. For further information on MIC interpretation and methodology, consult the most recent CLSI documents related to antifungal susceptibility testing of yeasts.

Clinical Limitation

Antifungal susceptibility results do not always correlate with clinical outcomes, as factors like immune status, infection site, drug pharmacokinetics, and biofilm presence can influence treatment success. CLSI breakpoints are limited to select yeast species and antifungal agents; for others, only MIC values are reported without interpretation. Some yeast species, show variable or dose-dependent susceptibility, complicating interpretation. Testing is performed under standardized conditions and may not reflect in vivo environments and evaluation of MIC may be impacted by methodology. Biofilm-related resistance is not assessed, and results from mixed or contaminated cultures may be unreliable. Additionally, the 24–48 hour turnaround time may delay therapy decisions. Results should be interpreted in the context of the full clinical picture.

Clinical Reference

Pappas PG, Kauffman CA, Andes DR, et al. Clinical Practice Guideline for the Management of Candidiasis: 2016 Update by the Infectious Diseases Society of America. Clin Infect Dis. 2016;62(4):e1-e50 CLSI. Performance standards for antifungal susceptibility testing of yeast, 3rd ed. CLSI supplement M27M44S. Clinical Laboratory Standards Institute; 2022. CLSI. Reference Method for Broth Dilution Antifungal Susceptibility Test of Yeasts, 4th ed. CLSI standard M27. Clinical Laboratory Standards Institute; 2017.