Fungal Identification and Antifungal Testing

Fungal identification and antifungal testing are the laboratory methods that detect and name the yeasts and moulds causing human infection and measure their susceptibility to antifungal agents. Diagnostic mycology combines microscopy and culture with antigen and antibody detection, molecular methods, and proteomic identification, and adds in vitro susceptibility testing for antifungal agents.

Definition

Fungal identification is the laboratory determination of the species of a yeast or mould from a clinical specimen, by microscopy, culture, serology, molecular, or proteomic methods; antifungal testing is the in vitro measurement of a fungal isolate's susceptibility to antifungal agents.

Scope

The entry covers direct microscopy and culture, antigen and antibody (serologic) detection, nucleic-acid and proteomic identification of fungi, and antifungal susceptibility testing with its interpretive breakpoints. It addresses these as laboratory methods and does not provide antifungal selection or dosing for patients.

Core questions

• Is a fungus present in this specimen, and what species is it?
• Which methods - microscopy, culture, antigen or antibody detection, molecular assays, or mass spectrometry - best detect and identify it?
• Is the isolate susceptible to relevant antifungal agents, and how is that measured and interpreted?
• What are the limits of fungal diagnostics in sensitivity, speed, and standardisation?

Key concepts

• Direct microscopy (KOH, calcofluor, histopathology)
• Fungal culture and morphology
• Antigen detection (e.g. galactomannan, cryptococcal antigen, beta-D-glucan)
• Antibody (serologic) testing
• Molecular and proteomic identification
• MALDI-TOF mass spectrometry of fungi
• Antifungal susceptibility testing and breakpoints
• Yeasts versus moulds

Mechanisms

Diagnostic mycology detects fungi by visualising them in specimens with stains and microscopy, by growing them in culture and reading colony and microscopic morphology, and by culture-independent methods. Antigen assays detect circulating fungal cell-wall or capsular components, while antibody tests detect the host response; together with molecular assays and proteomic identification these extend detection to organisms that are slow-growing or hard to culture (Kozel & Wickes, 2014). Species-level identification increasingly uses MALDI-TOF mass spectrometry, which matches a fungal isolate's protein spectrum to a reference database (Clark et al., 2013). Antifungal susceptibility testing then measures the inhibitory activity of antifungal agents against an isolate and interprets it against breakpoints; standardising this testing and relating it to outcomes has been an evolving effort (Rex & Pfaller, 2002). Professional guidelines situate these laboratory methods within the broader diagnostic evaluation of fungal disease (Pappas et al., 2016).

Clinical relevance

Fungal identification and susceptibility results contribute to clinical reasoning about invasive and superficial fungal infection and inform stewardship of antifungal agents. This entry describes how those laboratory results are produced and interpreted; it is reference material and is not a basis for selecting or dosing antifungal therapy for an individual, which is addressed by clinical guidelines and the treating team.

Epidemiology

Diagnostic mycology supports surveillance of fungal pathogens and emerging antifungal resistance, including shifts in the species responsible for infections such as candidiasis. Guideline documents reflect how laboratory diagnostics and susceptibility data inform management at the population and individual level (Pappas et al., 2016).

History

Mycological diagnosis long depended on microscopy and slow culture with morphological identification. Antigen and antibody assays, then molecular and proteomic methods, broadened and accelerated detection (Kozel & Wickes, 2014), while antifungal susceptibility testing matured from a research tool toward a standardised clinical assay over the late twentieth and early twenty-first centuries (Rex & Pfaller, 2002).

Seminal works

• kozel-2014
• rex-2002
• pappas-2016

Frequently asked questions

How are fungi detected when culture is slow or negative?

Laboratories use culture-independent methods - antigen assays (such as cryptococcal antigen, galactomannan, or beta-D-glucan), antibody tests, and molecular assays - to detect fungi that grow slowly or cannot be cultured from a given specimen (Kozel & Wickes, 2014).

What is antifungal susceptibility testing?

It is the in vitro measurement of whether an antifungal agent inhibits a fungal isolate, interpreted against breakpoints. Standardising this testing and linking it to clinical outcomes has been a gradual process (Rex & Pfaller, 2002).