Antifungal susceptibility testing
المؤلف:
Patricia M. Tille, PhD, MLS(ASCP)
المصدر:
Bailey & Scotts Diagnostic Microbiology
الجزء والصفحة:
13th Edition , p783
2025-12-11
65
Antifungal susceptibility tests are designed to provide information that helps the physician select the appropriate antifungal agent to treat a specific infection. Although antifungal susceptibility testing perhaps has not advanced as far as methods for determining the susceptibility of bacteria to antimicrobial agents, significant progress has been made. Substantial efforts have attempted to develop a standardized method that is reproducible among different laboratories. All of the technical variables in the testing process have been standardized, and efforts are underway to develop interpretative guidelines for different antifungal agents.
The Clinical Laboratory Standards Institute (CLSI) sets the standards for antifungal susceptibility testing. The current guidelines for these tests are provided in the following three documents, which are available on the CLSI website (www.clsi.org):
• Document M27-A3, Reference Method for Broth Dilution Antifungal Susceptibility Testing of Yeasts, Approved Standard, 3rd edition. This document covers requirements for use of the broth microdilution method. The standards for susceptibility testing are very specific about the inoculum size, test medium, incubation time and temperature, and end point of yeasts that cause invasive fungal infections.
• Document M38-A2, Reference Method for Broth Dilution Antifungal Susceptibility Testing of Filamentous Fungi, Approved Standard, 2nd edition. This standard is a microdilution method for molds that cause invasive and cutaneous infections.
• Document M44-A2, Method for Antifungal Disk Diffusion Susceptibility Testing of Yeasts, 2nd edition, Approved Guideline. This standard provides methodology for disk diffusion testing for Candida spp., including quality control and interpretation guidelines. Also see the International Supplement M44S3.
It must be emphasized that the methodology and interpretation of antifungal susceptibility tests continue to evolve, and the laboratory should check for updated standards or a regular basis. Antifungal susceptibility tests are costly and time-consuming, but they may have value in the following circumstances:
• Determining antibiograms for isolates in an institution
• Aiding the management of patients with refractory oropharyngeal candidiasis
• Aiding the management of patients with invasive candidiasis caused by non-albicans Candida spp.
when the use of the azoles is in question
The interpretative breakpoints for fluconazole, itraconazole, and flucytosine are based on experience in treating patients with mucosal infections, but they also appear to be consistent with information assembled for invasive infections. Problems that complicate the inter pretative guidelines include:
• Patient’s physical condition (i.e., immunologic status)
• Type of infection and the drug’s ability to penetrate a closed space (in the case of an abscess)
• Dose of the drug and its pharmacokinetics
• Susceptibility testing method used and serum level of drug administered
Isolates of the same species may exhibit differences in minimum inhibitory concentrations (MICs) because of previous exposure to antifungal agents and/or acquisition of a genetic mechanism of resistance. For example, some isolates of Candida glabrata show susceptibility to fluconazole, but others do not. CLSI’s interpretative guidelines should be followed whenever possible, but anecdotal experience also is useful.
Despite the problems associated with antifungal susceptibility testing, many physicians believe that these tests are important for selecting an appropriate antifungal agent and as a method to detect the development of resistance of certain organisms during chemotherapy. A laboratory that is not equipped to perform CLSI methods, or validated equivalent methods, for susceptibility of clinically important fungal isolates should be prepared to send the isolate to a reference laboratory for testing. Amphotericin B, 5-fluorocytosine, ketoconazole, itraconazole, voriconazole, and fluconazole are common anti fungal agents that traditionally have been tested; the newer antifungal agents may now be added to this list. For those interested in further information on this topic, the susceptibilities of many of the newer antifungal agents to some of the more challenging fungal pathogens have been published in a minireview by Lass Florl et al.
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