Antifungal susceptibility testing is a laboratory procedure that determines how sensitive a fungal infection is to various antifungal medications. It helps to identify which antifungal drugs are likely to be effective in treating a specific fungal infection. This testing guides healthcare professionals in selecting appropriate treatments, particularly as antifungal resistance becomes a growing concern. The goal is to predict the likelihood of successful treatment and to avoid using drugs that might not work.
Understanding Antifungal Susceptibility Testing
Antifungal susceptibility testing measures an antifungal agent’s ability to prevent the growth of a specific fungal organism. Performed in a controlled environment, it helps determine the concentration of a drug needed to inhibit the fungus causing an infection.
The importance of this testing has grown due to rising antifungal resistance in species like Candida glabrata, Candida auris, and Aspergillus fumigatus. Knowing which drugs a fungus is susceptible to helps prevent ineffective medication use and guides optimal therapy selection. This information is valuable for managing patient care and understanding broader trends in drug resistance.
Antifungal susceptibility testing is often ordered in specific clinical situations. It is typically considered for severe or invasive fungal infections, where the stakes of treatment failure are high. The test is also used when an infection does not respond to initial treatment, when a patient is immunocompromised, or in cases of recurrent infections.
Common Testing Methods
Antifungal susceptibility testing involves exposing fungal organisms to different concentrations of antifungal drugs to observe growth inhibition. Standardized procedures from organizations like the Clinical and Laboratory Standards Institute (CLSI) and the European Committee on Antimicrobial Susceptibility Testing (EUCAST) help ensure consistent results across laboratories.
One widely used method is Broth Microdilution (BMD), considered a gold standard. It involves preparing serial two-fold dilutions of antifungal drugs in a liquid growth medium within 96-well microtiter plates. A standardized amount of the fungal organism is added to each well, and the plates are incubated. The Minimum Inhibitory Concentration (MIC) is then determined as the lowest drug concentration that visually prevents fungal growth.
The Disk Diffusion method is a simpler, more qualitative approach. A standardized fungal inoculum is spread uniformly over an agar plate. Paper disks impregnated with specific concentrations of antifungal drugs are then placed on the agar surface. As the drug diffuses, a zone of inhibition—a clear area where fungal growth is prevented—forms around the disk. The size of this zone indicates the fungus’s susceptibility.
The Etest, or gradient diffusion method, combines aspects of both broth microdilution and disk diffusion. It uses a plastic strip with a continuous concentration gradient of an antifungal agent. When the strip is placed on an agar plate inoculated with the fungus, the drug diffuses into the agar. An elliptical zone of growth inhibition forms, and the MIC is read directly from the scale on the strip where the ellipse intersects it.
Deciphering Test Results
The primary result from antifungal susceptibility testing is the Minimum Inhibitory Concentration (MIC). The MIC is the lowest concentration of an antifungal drug that prevents visible fungal growth. This value is typically expressed in micrograms per milliliter (µg/mL) or milligrams per liter (mg/L). A lower MIC value generally suggests that less of the drug is needed to inhibit the fungal growth, indicating greater effectiveness.
To interpret MIC results, laboratories use standardized numerical values called breakpoints. Established by organizations like CLSI and EUCAST, these breakpoints serve as thresholds to categorize a fungus’s susceptibility. Breakpoints are specific to the fungal species and the antifungal drug being tested.
Based on the MIC relative to breakpoints, results are categorized into three interpretive groups: “Susceptible (S),” “Intermediate (I),” and “Resistant (R).” Susceptible indicates the fungus will likely respond to standard drug doses. Intermediate means the response may be uncertain or require a higher dose. Resistant signifies the fungus is unlikely to respond, even at higher doses. A healthcare professional must interpret these laboratory findings within a patient’s overall clinical context.
Guiding Treatment Decisions
Healthcare providers use antifungal susceptibility testing results to select the most suitable and effective medication for a patient’s infection. The interpretive categories (Susceptible, Intermediate, Resistant) directly inform treatment choices. If a fungus is “Susceptible” to a drug, it’s a strong candidate; if “Resistant,” the clinician will likely avoid it.
This personalized approach is important for severe or drug-resistant fungal infections. Knowing the susceptibility profile helps avoid ineffective treatments, reducing patient exposure to unnecessary side effects and potentially shortening recovery times. This targeted therapy also helps prevent further antifungal resistance.
By promoting responsible antifungal medication use, accurate testing contributes to better patient outcomes. It ensures patients receive the right drug, at the appropriate dose and duration, aligning with antimicrobial stewardship principles. This practice benefits individual patients and supports broader public health efforts to combat drug-resistant fungi.