Two-site immunoradiometric assay for detection of Plasmodium falciparum antigen in blood using monoclonal and polyclonal antibodies.

Three systems of immunoradiometric assays (IRMAs), a two-site monoclonal antibody sandwich IRMA (MAb-IRMA), two-site polyclonal antibody-monoclonal antibody sandwich IRMA (PAb-MAb-IRMA), and two-site polyclonal antibody sandwich IRMA (PAb-IRMA), were developed to detect low-grade infections with Plasmodium falciparum. The assays showed good correlation with parasitemia when tested against parasites from in vitro cultures (r = 0.996, 0.994, and 0.998 for MAb-, PAb-MAb-, and PAb-IRMA, respectively), with the ability to detect as few as 0.24, 0.67, and 1.82 parasites per 10(7) erythrocytes, respectively. The assays were specific for P. falciparum, since a serially diluted specimen from a patient with vivax malaria with an initial parasitemia of 0.8% and almost all of the undiluted specimens from five other vivax malaria patients were negative. The assays were performed on patients with falciparum malaria before and after treatment with antimalarial drugs. Before treatment, all 24 patients were positive by all three systems of two-site sandwich IRMAs. Two weeks after treatment, 81.8% (18 of 22) of the patients were positive by microscopic examination, but the IRMA positivity rates were 90.9% (20 of 22), 86.4% (19 of 22), and 81.8% (18 of 22) for MAb-, PAb-MAb-, and PAb-IRMA, respectively. Four weeks after treatment, all 19 patients were negative by microscopic examination, but 52.6% (10 of 19) of the patients were still positive with MAb- and PAb-MAb-IRMA and 31.6% (6 of 19) were positive with PAb-IRMA. Comparison between the three systems of IRMA showed that the MAb-IRMA was superior to the other two systems for three reasons. First, it gave a lower count when tested with blood from healthy individuals. Second, it gave a higher count when tested with blood from patients with falciparum malaria. Third, it gave better correlation with parasitemia when blood from falciparum malaria patients was tested. MAb-IRMA is recommended for use for the detection of low-grade P. falciparum infection.