SEMIQUANTITATIVE MEASUREMENT OF PLASMODIUM-FALCIPARUM ANTIGEN PFHRP2 IN BLOOD AND PLASMA

Plasmodium falciparum histidine rich protein 2 (PfHRP2) antigen was me asured semi-quantitatively in whole blood, plasma, and supernatants an d red blood cells of cultures in vitro using the dipstick Para-Sight(T M) test and also by a quantitative antigen-capture enzyme-linked immun osorbent assay (ELISA). In vitro, PfHRP2 was secreted mainly during th e second half of the asexual cycle with a marked rise during schizont development and rupture. The total PfHRP2 secreted before schizogony c orresponded to approximately 4% of that contained in the red blood cel ls. In samples from 55 patients with acute falciparum malaria, the lev el of detection by ELISA corresponded to parasitaemias of 100/mu L for whole blood and 1600/mu L for separated plasma. Whole blood PfHRP2 le vels were correlated significantly with admission parasitaemia (r=0.76 , P<0.0001) and the stage of parasite development (r=0.43, P<0.01). Al though whole blood PfHRP2 concentrations were higher in severe malaria , plasma concentrations of PfHRP2 were considerably higher in severe m alaria (median titre 1:320, range zero to 1:1280) than in uncomplicate d 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 uncomplica ted 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 d ipstick correlated well with more precise measurement of optical densi ty in the ELISA (r=0.88, P<0.0001). These results suggest that measure ment of PfHRP2 in plasma could provide an alternative approach to the assessment of the parasite biomass, and thus prognosis, in severe mala ria, and that this could be done simply by using the currently availab le dipsticks.