Performance of rapid malaria Pf antigen test for the diagnosis of malaria and false-reactivity with autoantibodies.

Recently introduced rapid nonmicroscopic immunocapture assays for the diagnosis of malaria infection are being evaluated for their sensitivity and specificity in various epidemiological settings. A Plasmodium falciparum histidine-rich protein-2 (PfHRP-2)-based assay (ICT malaria Pf assay) was evaluated for its performance and compared to that of Giemsa-stained thick blood film microscopy. Of the 515 patients tested, 163 were positive for malaria parasites on thick blood film microscopy: 87 were infected with P. vivax; 63 with P. falciparum; 1 with P. malariae; and 12 with both P. falciparum and P. vivax. The ICT assay detected 53 P. falciparum infections and, as expected, failed to detect all but one case of P. vivax. Three cases of mixed infections were also not detected by this assay. The performance of the ICT assay in diagnosing P. falciparum infection was comparable to that of microscopy. The sensitivity of the ICT assay was 82% and the specificity 99.0%. The ICT assay also detected 4 false-positive cases. These patients reported treatment with chloroquine in the previous 2-5 weeks. The specificity of the assay was evaluated in different groups of patients, who had tested negative for malaria infection by microscopy. These patients were selected from different disease groups: rheumatoid arthritis; hepatitis C; toxoplasmosis; schistosomiasis; and hydatid disease. Of the 225 patients studied, 133 were positive for rheumatoid factor. Thirty-five (26%) of the 133 patients had false positive-reactions with the ICT assay, while only four had false positive-reactions with the OptiMAL test. After the rheumatoid factor was absorbed 33 of the 35 false-positive specimens were negative when retested with the ICT assay. 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 microscopy. We conclude that new rapid nonmicroscopic methods for the diagnosis of malaria that complement or support blood film microscopy would be of great use in the diagnosis and treatment of patients with malaria and also in epidemiological studies.