Immunoblot assay using recombinant antigens as a supplemental test to confirm the presence of antibodies to Trypanosoma cruzi

Abstract

The diagnosis of chronic Chagas' disease is generally made by detecting antibodies to Trypanosoma cruzi. Most conventional serological tests are based on lysates of whole parasites or semipurified antigen fractions from T. cruzi epimastigotes grown in culture. The occurrence of inconclusive and false-positive results has been a persistent problem with the conventional assays, and there is no universally accepted gold standard for confirmation of positive test results. We describe here an immunoblot assay for detecting antibodies to T. cruzi in which four chimeric recombinant antigens (rAgs), designated FP3, FP6, FP10, and TcF, are used as target antigens. Each of these rAgs is composed of several antigenically distinct regions and includes repetitive as well as nonrepetitive sequences. Each rAg is coated as a discrete line on a nitrocellulose strip. Assay sensitivity was assessed by testing 345 specimens known to be positive for antibodies to T. cruzi. All 345 of these samples showed two to four reactive test bands in addition to the three on-board control bands that are on each strip. Assay specificity was determined by testing 500 specimens from random U.S. blood donors, all of which gave negative results. Based on the results obtained in this study, we propose the following scheme for interpretation of test results: (i) no bands or a single test band = a negative result; (ii) two or more test bands with at least one band showing intensity of 1+ or higher = a positive result; and (iii) multiple faint test bands (+/-) = indeterminate result. Based on this scheme, the prototype immunoblot assay showed sensitivity of 100% (n = 345) and specificity of 100% (n = 500). Additionally, all 269 potentially cross-reacting and T. cruzi antibody-negative specimens tested negative in our immunoblot assay. The rAg-based immunoblot assay has potential as a supplemental test for confirming the presence of antibodies to T. cruzi in blood specimens and for identifying false-positive results obtained with other assays.

FIG. 1.

Typical results obtained with the Abbott immunoblot assay. Strip 1, negative control, showing three bands in the on-board control section (top); strip 2, T. cruzi antibody-positive control, showing four test bands (bottom) in addition to the three on-board control bands; strips 3 to 6, samples showing one to four test bands in addition to the on-board control bands.

FIG. 2.

Distribution of readings, in terms of the number of positive test bands present, obtained by testing 345 T. cruzi antibody-positive specimens in the Abbott immunoblot assay.

FIG. 3.

Algorithm for interpreting readings in the Abbott immunoblot assay. The bands referred to in the chart are the test bands, which are comprised of the recombinant antigens listed in Table 1. For each individual assay to be valid, the three on-board control bands must be present.