The sickening reality of malaria diagnosis
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.
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