Blastomycosis laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: ; Vidit Bhargava, M.B.B.S [2]

Overview

Once suspected, the diagnosis of blastomycosis can usually be confirmed by demonstration of the characteristic broad based budding organisms in sputum or tissues by KOH prep, cytology, or histology.

Laboratory Findings

Commonly performed tests include:

• KOH preparation - shows a broad based budding yeast as shown in the microscopic picture above. sputum, blood, pleural and other body fluids may be used, however this process has a low clinical yield.
• Tissue biopsy of skin or other organs may be required in order to diagnose extra-pulmonary disease. A granulomatous inflammation might be suggestive of fungal presence but is not diagnostic.
• Commercially available urine antigen testing appears to be quite sensitive in suggesting the diagnosis in cases where the organism is not readily detected. It appears to be more helpful than serum antigen testing.
• While culture of the organism remains the definitive diagnostic standard, its slow growing nature can lead to delays in treatment of up to several weeks. Culture on dextrose sabourd agar at 37ºC can be used for diagnosis. Highest diagnostic yield is of bronchoscopy derived fluid, followed by sputum. Real time PCR is being experimentally tested for direct diagnosis from culture or tissue.
• Serological testing is limited in utility by the fact that there is a considerable overlap with other fungal antigens.

However, sometimes blood and sputum cultures may not detect blastomycosis; lung biopsy is another option, and results will be shown promptly.

Microscopy

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  Broad based budding yeast

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  Smear from a foot lesion showing a Blastomyces dermatitidis yeast cell.