Discontinuation of DNA Probes for Identification of Dimorphic Fungi Growing in Culture: What's a Lab To Do?

The most common dimorphic fungi isolated from clinical specimens in North America are Coccidioides immitis, Coccidioides posadasii, Blastomyces dermatitidis species complex, and Histoplasma capsulatum. These organisms are typically definitively identified at reference or public health laboratories, as they are risk group 3 (RG3) pathogens requiring additional biosafety considerations compared to risk group 2 (RG2) pathogens. Reference and public health laboratories have been using organism-specific DNA probes since the early 1990s as the primary method of confirming the identification of morphologically suspect dimorphic fungi growing in culture. At the end of November 2021, manufacturing of these probes was discontinued, leaving clinical laboratories responsible for dimorphic fungus identification with the task of validating and implementing a new identification method for these pathogens. Here, we discuss alternatives to DNA probes for identification of Coccidioides spp., B. dermatitidis species complex, and H. capsulatum growing in culture, including the strengths and limitations of each method.