Unpredictability of commercially available exoantigen culture confirmation tests in confirming the identity of five Blastomyces dermatitidis isolates

Colonial and microscopic features of five fungal isolates from three patients suggested Blastomyces dermatitidis. Extracts from the mold forms of all isolates were tested on several occasions with commercially available Exoantigen immunodiffusion culture-confirmation test reagents and Nolan reagents. All three isolates from patient 1 were negative on four separate attempts with Exoantigen reagents using conventional ("slant") and "broth" extraction methods, and were also negative on one attempt with the Nolan reagents. The isolate from patient 3 was negative on three attempts using both reagent kits. The isolate from patient 2 was negative on four of five attempts with Exoantigen test reagents and positive on one attempt with Nolan reagents. Commercially prepared chemiluminescence-labeled DNA probes (Gen-Probe, San Diego, CA) directed at ribosomal RNA from B. dermatitidis and Histoplasma capsulatum confirmed all five isolates as B. dermatitidis. The cost and labor of the exoantigen and DNA Probe culture confirmation tests were evaluated. New methods for confirming the identity of cultural isolates of B. dermatitidis that are sensitive, specific, and commercially available are greatly needed.