Immunocapture diagnostic assays for malaria using Plasmodium lactate dehydrogenase (pLDH)

We have developed two diagnostic assays based on the specific detection of Plasmodium lactate dehydrogenase (pLDH) activity. These assays exploit a panel of monoclonal antibodies that capture the parasite enzyme and allow for the quantitation and speciation of human malaria infections. An immunocapture pLDH activity assay (ICpLDH) allows for the rapid purification and measurement of pLDH from infected blood using the NAD analog APAD, which reacts specifically with Plasmodium LDH isoforms. An immunochromatographic test (the OptiMAL assay) was also formatted and allowed the detection of parasite infections of approximately 200 parasites/microl of blood. By using a combination of antibodies, both tests can not only detect but differentiate between P. falciparum and non-P. falciparum malaria. Both assays show a sensitivity comparable with other commercial nonmicroscopic tests; importantly, we found very few instances of false-positive samples, especially with samples from patients recently cleared of malaria infection. Furthermore, we find that when one uses the quantitative ICpLDH assay, the levels of pLDH activity closely mirror the levels of parasitemia in both initial diagnosis and while following patient therapy. We conclude that diagnostic tests based on the detection of pLDH are both sensitive and practical for the detection, speciation, and quantitation of all human Plasmodium infections and can also be used to indicate drug-resistant infections.