How RDTs work against malaria
Malaria rapid diagnostic tests (RDTs) aid in the diagnosis of malaria by providing evidence of the presence of malaria parasites in human blood. RDTs are an alternative to clinic-based diagnosis or microscopy, particularly where good quality microscopy services cannot be readily provided.
There are variations between products, such as objectives and formats, although the principles of testing are similar. Malaria RDTs detect specific antigens (proteins) produced by malaria parasites in the blood of infected people. Some RDTs can detect only one species (Plasmodium falciparum or P. vivax), while others detect multiple species (P. falciparum, P. vivax, P. malariae, and P. ovale). Blood for the test is commonly obtained from a finger prick.
RDTs are lateral flow immunochromatographic antigen detection tests, which rely on the capture of dye-labelled antibodies to produce a visible band on a nitrocellulose strip, often enclosed in a plastic casing, called a cassette. With malaria RDTs, the dye-labelled antibody first binds to a parasite antigen, and the resulting complex is captured on the strip by a band of bound antibodies, forming a visible line (T-test line) in the window of the strip. results. A control line (control line C) provides information on the integrity of the antibody-dye conjugate but does not confirm the ability to detect parasite antigens.
Malaria Rapid Test Cassette
Inside the cassette is a strip made of filter paper and nitrocellulose. Typically, a drop of blood is added to the RDT through one hole (A; sample well) and then a number of drops of buffer, usually through another hole (B; buffer well). The buffer carries the blood along the PDR.
Mode of action of common malaria PDR format
1. The first step in the test procedure is to mix the patient’s blood with a lysing agent in a test strip or well. This breaks down the red blood cells and releases more protein from the parasite.
2. Dye-labeled antibody, specific for the target antigen, is present on the bottom end of the nitrocellulose strip or in a plastic well provided with the strip. An antibody, also specific for the target antigen, binds to the strip in a thin (test) line, and an antibody specific for the labelled antibody or antigen binds to the control line.
3. The blood and buffer, which have been placed on the strip or in the well, are mixed with the labelled antibody and drawn onto the strip along the lines of the bound antibody.
4. If the antigen is present, any labelled antibody-antigen complex will be trapped and accumulate on the test line. The excess-labelled antibody is trapped and accumulates in the control line. A visible control line indicates that the labelled antibody has traversed the full length of the strip, beyond the test line, that at least some of the free antibody remains conjugated to the dye and that some of the capture properties of the antibodies remain intact.
5. The intensity of the test band will vary with the amount of antigen present, at least at low parasite densities (antigen concentration), as this will determine the number of dye particles that will accumulate on the line. The intensity of the control band may decrease at higher parasite densities since the test band will have captured much of the labelled antibody before reaching the control.