Plasmodium falciparum histidine-rich protein 2-based immunocapture diagnostic assay for malaria: Cross-reactivity with rheumatoid factors

Recently introduced rapid nonmicroscopic immunocapture assays for the diagn osis of malaria infection are being evaluated for their sensitivity and spe cificity in various epidemiological settings. A Plasmodium falciparum histi dine-rich protein 2 (PfHRP-2)-based assay (ICT) and a Plasmodium-specific l actate dehydrogenase test (OptiMAL) were evaluated for their specificities in different groups of patients who tested negative for malaria infection b y microscopy. The patients were selected from different disease groups: rhe umatoid arthritis, hepatitis C, toxoplasmosis, schistosomiasis, and hydatid disease. One hundred thirty-three of the 225 patients were positive for rh eumatoid factor. Thirty-five of the 133 (26%) rheumatoid factor-positive pa tients gave a false-positive reaction with the ICT assay, but only 4 of the se gave false-positive reactions with the OptiMAL test. Thirty-three of the 35 false-positive specimens became negative when repeat tested with the IC T assay after absorbing out the rheumatoid factor activity. Our study shows that the PfHRP-2-based ICT assay gave a false-positive reaction in 26% of the patients who had rheumatoid factors, but were negative for malaria by m icroscopy.