A colorimetric in vitro drug sensitivity assay for Plasmodium falciparum based on a highly sensitive double-site lactate dehydrogenase antigen-capture enzyme-linked immunosorbent assay

We report a double-site enzyme-linked lactate dehydrogenase immunodetection assay (DELI), a highly sensitive antigen-capture enzyme-linked immunosorbent assay, which proved to be more sensitive for the detection of Plasmodium falciparum than thick blood smears, as sensitive as the polymerase chain reaction, and probably more reliable. This technique can help to detect infra-microscopic parasitemias (one parasite in 10(6)-10(8) red blood cells) from biological samples, and being quantitative, provide a fast substitute to thick smears for epidemiologic purposes. The technique can also be used to measure the in vitro drug sensitivity of P. falciparum with greater ease, much greater speed, and simpler equipment than that required for the isotopic microtest. Results obtained with four antimalarial drugs upon 16 strains closely paralleled those obtained by the isotopic assay (R = 0.95). In contrast with the latter, much lower parasite densities could be tested in the DELI assay (as low as 0.005%), thereby extending the number of isolates that can be investigated. The ease of implementation and low cost of the DELI-microtest may contribute to a revived interest in using in vitro methods to survey resistance to antimalarial drugs, so as to better predict future in vivo drug failures and provide public health recommendations.