Mode of action of antigen-detecting malaria rapid diagnostic tests

A malaria rapid diagnostic test (RDT) is a lateral flow immunochromatographic device that detects protein (antigen [Ag]) derived from the blood stage of malaria parasites. Blood is usually obtained from a finger prick, in a similar way to that usually used for malaria microscopy. A small sample of blood, usually 5 to 20 microL, is placed on the RDT strip, or in a well of the cassette or card test device, and lysed to release the Ag from within red blood cells and parasites from within these cells (a variable amount of Ag is also present in the serum). After several minutes, the test produces a series of visible lines to signal the presence or absence of Ag in the blood sample by the mechanism outlined below:
(A) Dye-labeled antibody (Ab), specific for the target Ag, is present on the lower end of the nitrocellulose strip or in a well provided by a casing covering the strip. Ab, specific for another epitope on the target Ag, is bound to the strip in a thin (test) line, and Ab specific for the labeled Ab is bound at the control line.
(B) Blood and buffer, which have been placed on the strip or in the well, are mixed with labeled Ab and are drawn up the strip across the lines of bound Ab.
(C) If Ag is present, labeled Ab will be trapped on the test line. Other labeled Ab is trapped on the control line. If sufficient labeled Ab accumulates, the dye labels will become visible to the naked eye as a narrow line.