Immunoradiometric assay for detection of filarial antigens in human serum

The immunoradiometric assay (IRMA) for detection of filarial antigens in the serum of patients infected with Brugia malayi (Bm) or the closely related filarial parasite Wuchereria bancrofti (Wb) was investigated, and its performance and clinical utility were examined. Reference sera prepared by the addition of crude Bm antigen (BmA) to negative control human sera provided a reproducible reference curve. The IRMA displayed acceptable precision and reproducibility. Agreement between dilutions (parallelism) was good in sera without specific antibody, but the presence of even modest levels of antibody resulted in nonparallelism in about one-half of the tested sera from endemic areas. Significant reduction in detectable BmA occurred when low levels of specific antibody (<1 μg/ml) were added to BmA containing sera. Thus, antibody interference limited absolute quantitation of antigen in the IRMA. Results were therefore expressed in a semiquantitative manner by using the mean + 3 SD of the binding of nonexposed human sera as the positive threshold. The frequency of reliable filarial antigen detection in individuals from the Wb endemic areas of India and the South Pacific was the following: microfilaremia, 15 out of 15; elephantiasis, 2 out of 18; tropical pulmonary eosinophilia, 2 out of 8. These findings show clearly that a two-site IRMA can effectively detect circulating antigen (and thus be diagnostic of infection) in a great many patients with filariasis, but to enhance the sensitivity of the assay to the point where all patients can be diagnosed, a number of suggested modifications will be necessary.