Serologic response in blastomycosis: diagnostic value of double immunodiffusion assay.

Double immunodiffusion (DID) is a useful, inexpensive antibody screening test that is more specific than complement fixation for blastomycosis. Seven of 10 patients with proved blastomycosis had positive results on DID assays for antibody to the A antigen of Blastomyces dermatitidis. Sputum cytologic examination and skin or other tissue biopsies may be diagnostic if the organism is demonstrated. Culture remains the sina qua non of diagnosis, and a negative DID test result for A precipitin does not rule out this infection. Follow-up serologic studies of patients in remission and in relapse should be undertaken to determine the evolution of the immune response to the A antigen.