The Leeds Teaching Hospitals NHS Trust


Yeast Identification and Sensitivity

Yeast Identification and Sensitivity

Culture of yeasts should ideally be sent on a Sabouraud's agar slope in a bijou or universal.

 Sabourauds agar slope






Identification of yeasts and assessment of susceptibility to antifungals.


Identification, usually to species level using a combination of morphological and nutritional/enzymatic tests.  Molecular identification is carried out for those isolates which cannot be identified using phenotypic tests.

Susceptibility testing by CLSI M44A disc diffusion (fluconazole) or microbroth dilution (fluconazole, itraconazole, voriconazole, amphotericin B, flucytosine, caspofungin, posaconazole, anidulafungin and micafungin). Specific antifungal(s) tested depend on the identity and source of the isolate and the clinical details supplied. Microbroth dilution testing is undertaken where indicated by isolate identity, disc diffusion results, or where requested specifically. The identity of the yeast isolate is always confirmed or carried out on isolates sent for sensitivity testing.


Identity of the yeast, usually to species level, sensitivity to one or more antifungal agents

Susceptibility testing results, Clinical breakpoints (CBP) and Epidemiological cutoff values (ECV)

When an antifungal is reported as being "Susceptible" it means that it is likely the patient will respond to this antifungal at standard doses.

When an antifungal is reported as being "Susceptible dose-dependant" or "Intermediate" it means it is likely the patient will respond to this antifungal at higher than standard doses.

When an antifungal is reported as being "resistant", it means the patient is unlikely to respond to this antifungal.

Clinical breakpoints are defined as the highest minimum inhibitory concentration (MIC) at which a full response can normally be expected (in typical cases of infection with therapeutic drug levels) and is based on outcome data in clinical trials and other studies.  There are relatively few fungus/antifungal combinations for which this data is available. Susceptible-dose dependant or intermediate indicates response can normally be expected at a higher than standard dose. Resistance indicates that there is a relatively high likelihood that a patient will not have a positive outcome on that antifungal and it is usually advisable to avoid using it.

Epidemiological cutoff values are the highest MIC seen in wild type (WT) populations of the species that have not been exposed to the antifungal, and where the MIC is above the WT, it is considered Non-wild type and indicates that there is an increased likelihood that a fungus will not be inhibited by the antifungal in question and that an infection will not respond to that antifungal. However, there is no evidence available in terms of clinical outcomes on which to base a CBP.

For both CBPs and ECV it is important to state that outcomes from  any fungal infection treated with any antifungal will be a function of the fungal phenotype, the pharmokinetics of the antifungal in the patient being treated and the response of the patient’s immune systems. CBPs and ECVS simply provide useful guidance in choosing a therapeutic option.

The CBPs and ECVs for our asays are based on the Clinical Laboratory Standards Institute (CLSI) methodology or assays validated by this methodology.

Where there are CBPs susceptibility results are reported as "susceptible", "susceptible dose dependant", "intermediate" or "resistant".  Where there are ECVs, wild type , results are reported as "susceptible", and non-wild type results are reported as "resistant". MICs are available for some isolates on request.

Please feel free to contact mycology to discuss any sensitivity results.