2035. Detection of Blastomyces dermatitidis Antigen in Urine Using a Novel Quantitative Enzyme-linked Immunosorbent Assay

BACKGROUND: Detection of Blastomyces dermatitidis antigen (BdAg) in clinical specimens offers a rapid and non-invasive means to both diagnose blastomycosis and monitor patient response to therapy. There are currently no BdAg detection assays commercially available and the majority of BdAg testing is performed at a single reference laboratory (MiraVista Diagnositcs [MVDx], Indianapolis, IN). Here, we evaluated a novel, quantitative enzyme-linked immunosorbent assay (ELISA) based on a unique rabbit monoclonal antibody for detection of B. dermatitidis polysaccharide antigens in urine (Aliquot LLC, Gorham, Maine). METHODS: Clinical residual urine specimens collected from 86 unique patients with a previously negative (n = 63) or positive (n = 23) result by the MVDx Blastomyces Ag Quantitative EIA were evaluated by the Aliquot BdAg ELISA. Clinical information was available for five of these patients. In addition, analytical specificity was evaluated using 15 residual urine samples positive for Streptococcus pneumoniae (n = 5), Legionella pneumophila (n = 5) or Histoplasma capsulatum (n = 5) antigens. RESULTS: The Aliquot BdAg ELISA showed 95.7% (22/23), 96.8% (61/63) and 96.5% (83/86) positive, negative and overall agreement with the MVDx BdAg EIA, respectively. Seventeen of the 22 samples positive for BdAg by both assays resulted positive by a H. capsulatum antigen ELISA (IMMY, Norton, OK). Of the five well-characterized patients, one was diagnosed with blastomycosis based on a positive B. dermatitidis immunodiffusion result; this patient was positive by both BdAg assays. All urine samples positive for S. pneumoniae or L. pneumophila antigen were negative by the Aliquot BdAg ELISA, while all five samples positive by the IMMY H. capsulatum urine antigen ELISA were also positive by the Aliquot BdAg assay. CONCLUSION: The Aliquot BdAg ELISA demonstrated excellent agreement with the MVDx BdAg EIA. Cross-reactivity between B. dermatitidis and H. capsulatum antigen detection assays has been previously established and is a notable limitation to the Aliquot BdAg assay. Further evaluation of this assay using specimens from well-characterized patients with and without blastomycosis is warranted. DISCLOSURES: All authors: No reported disclosures.