RDT test: design and function

The basic principle is:

If a patient has a malaria infection:
(1) A blood sample will be lysed in buffer to release parasite antigens, and expose them to dye-labelled antibodies
(2) The labelled antibodies will bind to parasite proteins forming an antigen-antibody complexes.
(2) These complexes are then captured by a second antibody immoblised on a nitro-cellulase to form a visible band.

Test formats vary, but share common principles - this is illustrated for a single parasite antigen below.

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(1) In a typical test the blood sample is loaded into one window, and a lysis buffer in a second window. The lysed blood and parasite antigens then meet a parasite-specific antibody labelled with dye (show strip and mixing of agents = A). (2) Where the parasite-antigen is present, the dye-labelled antibody will bind to that antigen to form a complex (binding=B). This dye-labelled antigen/antibody complex then diffuses along the strip until it encounters a second parasite-specific antibody immobilised as a band on the strip. This immobilised antibody “captures” the labelled antibody/antigen complex to form a visible line (capture 1 = C). (3) The remaining lysed sample (containing labelled antibody not bound to parasite antigen) continues to diffuse along the strip. And encounters a further immobilised antibody that captures it. This forms a control line which indicates that test has been successfully performed (capture 2 = D).