Dirofilaria immitis: Performance and standardization of specific antibody immunoassays for filariasis

The factors associated with the development, optimization, and validation of immunoassays for the detection of parasite-specific antibody in filariasis infections were investigated using the dog heartworm, Dirofilaria immitis as a model. We examined two assays, the Protein A solid-phase radioimmunoassay (SPRIA) and enzyme-linked immunosorbent assay (ELISA), for quantitation of specific antibody to the parasite in canine serum. Precision, reproducibility, and parallelism were examined using response-error relationships and precision profile analyses. A staphylococcal Protein A saturation analysis was applied to the standardization of IgG antiparasite antibody reference sera in weight per volume units (μg/ml). Using the mean minimal detectable dose + 3 SD and an intraassay precision profile < 10% coefficient of variation (CV) as criteria for assay sensitivity, the SPRIA and ELISA displayed comparable positive thresholds of 1 μg/ml IgG anti-parasite antibody/ml of serum. Both assays also demonstrated good reproducibility (< 15% interassay CV) and parallelism (< 20% interdilutional CV) over their working ranges (SPRIA: 1-40 μg/ml; ELISA: 1-5 μg/ml). Specificity of each assay was enhanced by preadsorption of cross-reacting antibodies in canine serum (i.e., specific for Toxocara canis antigens) with solid-phase antigen prior to assay. Methods for comparing different immunoassay designs are considered in relation to the variables that influence the assays' performance characteristics.