Co-reactivity of plasmodial histidine-rich protein 2 and aldolase on a combined immuno-chromographic-malaria dipstick (ICT) as a potential semi-quantitative marker of high Plasmodium falciparum parasitaemia.

The combined immuno-chromographic-malaria dipstick (ICT) for the rapid diagnosis of malaria detects both Plasmodium falciparum (P.f.)-specific, histidine-rich protein 2 (HRP-2) and a plasmodial aldolase expressed by all Plasmodium species pathogenic to humans. ICT was applied in 674 febrile returnees from malaria-endemic regions attending our Tropical Diseases Unit. Microscopy confirmed malaria in 69/674 cases, of whom 67/69 had returned from Africa or Madagascar, and 2/69 from the Caribbean. Monoparasitic P.f. infection occurred in 52/69, mixed infection was due to P.f.+ P. ovale (P.o.) in 3/69, and P.f.+P. malariae (P.m.) in 1/69 cases. Monoparasitic P. vivax (P.v.) infection occurred in 8/69 , P.o. in 3/69, and P.m. in 2/69 cases . Whereas a positive HRP-2 band on the test was a highly sensitive indicator for P.f. infection (52/52 patients; sensitivity 100%), this was not the case for a positive aldolase band (25/52 patients; sensitivity 48.1%). Sensitivity of aldolase band for non-falciparum plasmodia was even lower: aldolase was positive in only 3/8 (37.5%) of patients with vivax malaria, and in 0/5 cases with P.o.- or P.m. infection. Co-reaction of both bands occurred more frequently in patients with P.f. parasitaemia of > or =40,000/microl (20/25, 80.0%) as compared to patients with P.f. parasitaemia <40,000/microl (5/27, 18.5%; P<0.00005), and to patients with mixed infection (P.f.+ P.o., P.f.+ P.m.: 2/4, 50.0%; diff. n.s.). In our series, co-reaction of HRP-2 and aldolase indicated monoparasitic falciparum malaria with high P.f. parasitaemia, rather than mixed infection. Whereas the aldolase band is not a reliable qualitative marker for malaria, co-reaction of HRP-2 and aldolase band may have a potential for indicating high parasitaemia in falciparum malaria.