Field study of an antigen-detection ELISA specific for Plasmodium falciparum malaria

We field-tested a specific antigen-detection enzyme-linked immunosorbent assay (ELISA) based on the histidine-rich protein of Plasmodium falciparum, in a district hospital in Thailand. The test was simple to perform, takes less than 3 h to complete, can deal with batches of sera, be read visually, and was 98.05% sensitive and 96.22% specific. However, 3 of 154 microscopically identified P. falciparum cases gave false negative ELISA results. One of these patients had been admitted to hospital for P. falciparum malaria 3 months previously and all 3 came from hyperendemic villages and were thought to have had previous episodes of malaria, possibly resulting in high titres of circulating blocking antibody. The test was more sensitive when whole blood was frozen and thawed before testing. This test is promising but requires further refinement to eliminate false negatives before it can be used safely for screening acutely ill patients for falciparum malaria. The sensitivity of this ELISA appears to be sufficiently high to consider it as a tool for blood donor screening in regions with a high prevalence of P. falciparum carriers with low parasitaemia. There is at present no satisfactory routine screening method for large numbers of blood donors for malaria.