Semi-quantitative measurement of Plasmodium falciparum antigen PfHRP2 in blood and plasma

Plasmodium falciparum histidine rich protein 2 (PfHRP2) antigen was measured semi-quantitatively in whole blood, plasma, and supernatants and red blood cells of cultures in vitro using the dipstick Para-Sight(TM) test and also by a quantitative antigen-capture enzyme-linked immunosorbent assay (ELISA). In vitro, PfHRP2 was secreted mainly during the second half of the asexual cycle with a marked rise during schizont development and rupture. The total PfHRP2 secreted before schizogony corresponded to approximately 4% of that contained in the red blood cells. In samples from 55 patients with acute falciparum malaria, the level of detection by ELISA corresponded to parasitaemias of 100/mu L for whole blood and 1600/mu L for separated plasma. Whole blood PfHRP2 levels were correlated significantly with admission parasitaemia (r=0.76, P<0.0001) and the stage of parasite development (r=0.43, P<0.01). Although whole blood PfHRP2 concentrations were higher in severe malaria, plasma concentrations of PfHRP2 were considerably higher in severe malaria (median titre 1:320, range zero to 1:1280) than in uncomplicated malaria (median titre 1:5, range zero to 1:80; P<0.0001). The ratio of whole blood to plasma PfHRP2 was lower in severe than in uncomplicated malaria (median 4, range 0.25 to 256, versus 64, range 4 to 1280; P<0.0001). With plasma samples the intensity of colour change on the dipstick correlated well with more precise measurement of optical density in the ELISA (r=0.88, P<0.0001). These results suggest that measurement of PfHRP2 in plasma could provide an alternative approach to the assessment of the parasite biomass, and thus prognosis, in severe malaria, and that this could be done simply by using the currently available dipsticks.