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Blastomyces antigen testing is species agnostic in Wisconsin

September 15 2020

In the last 3 years, decades of ‘knowledge’ about Blastomyces dermatitidis was upset by the finding of multiple cryptic species within the genus. B. gilchristii is more often associated with pulmonary disease without cutaneous infection, whereas B. dermatitidis and B. helicus more commonly cause skin lesions and other features of dissemination.  Blastomyces percursus and Blastomyces emzantsi are found in Africa. Given this significant shift in taxonomy, a question arises about diagnostic performance of non-culture based assays.

Klaire Laux and colleagues from the Marshfield Clinic Research Institute, Wisconsin (USA), examined the performance of MiraVista urine antigen testing for Blastomyces in 140 patients, using archived results. All cases were culture confirmed and speciated using ITS variation by SNP analysis or sanger sequencing. Sixty-three cases (45%) were caused by B. dermatitidis and 77 cases (55%) by B. gilchristii.

In 75 cases (54%), urine antigen EIA testing was ordered by physicians and in 35 cases (25%) multiple times (2-18 tests/case patient, mean 6). Of those patients tested, most were tested within 7 days of diagnosis (n=58, 77%) and received only one test (n=40, 53%). In these 75 cases, 16 cases were negative, demonstrating a 79% overall sensitivity rate. Testing was more likely to be positive if requested in the first week of diagnostic workup (p <0.0001). 

There was no difference between sensitivity related to the species of Blastomyces. Tests were more likely to be positive in cases of pulmonary disease than disseminated infection 81% vs 64%, but this was not statistically significant.