Each year, an estimated 350 million to 500 million people contract malaria, and about 1 million to 2 million of them die. Although Giemsa-stained light microscopy can reliably detect the blood parasite that causes the disease, most cases occur in countries that lack trained microscopists or sufficient capital to initiate a microscopy program. Rapid diagnostic tests could become a cheaper alternative, but their reliability must first be improved, wrote David Bell and colleagues from the World Health Organization’s Regional Office for the Western Pacific in Manila, Philippines.

Currently, rapid diagnostic tests are lateral immunochromatographic assays that consist of a nitrocellulose strip that may be encased in protective housing. A patient’s blood is placed on the strip and mixed with a dye-labeled antibody and buffer, and the resulting solution travels along the test strip. If the malarial parasite antigen is present, it becomes trapped on a test line that is visible to the naked eye.

In a review of malaria diagnosis, the authors call for the development of standards of measurement and more-specific assays that can withstand high temperatures. Existing rapid diagnostic tests can detect only certain antigens, the structure and expression of which can vary between individual parasite species. Furthermore, some tests deteriorate rapidly at higher temperatures, but malaria outbreaks occur in hot, tropical countries that lack the money for widespread, adequate temperature control. Finally, although a standard of 100 parasites per microliter of blood has been suggested, it requires comparison to the ratio of antigen to parasite density, information that is unattainable from the test itself.

The reviewers also suggest that manufacturers develop tests that are easy to use, that ensure quality control using panels standardized by enzyme-linked immunosorbent assays and that provide a method for the end user to ascertain whether the test still works properly.

In clinical trials, microscopy often is used to verify the performance of rapid diagnostic tests. The authors state that the quality of microscopy is a major determinant in these trials, as is the study population, and that industry should have standards for documentation and review. (Nature Reviews Microbiology, September 2006, pp. 682-695.