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RDT test: design and function

From MalariaETC


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Test formats share common principles, here we illustrate the most simple test format. A "single antigen" test recognising P.falciparum.


TEST PROCESS


Part 1 Essentially buffer is introduced through one window (marked A); this buffer contains lysing agents. The blood sample loaded into a second window (marked B). These two mix to cause lysis of red cells and any malarial parasites which then diffuse along the strip where they encounter labelled anti-malarial antibody that is not fixed to the strip (Ab). The mixing of antibody and lysed materials allows the antibodies bind to any malaria-specific proteins present in the sample forming immune complexes. The mixture of immune complexes, free antibody, and red cells proteins then continues to migrate along the strip driven by excess buffer (blue arrow).



The process is shown in more detail below: buffer (A) and blood sample (B) enter the strip where red cells and parasites are lysed. The lysed red cells and any parasite antigens then encounter the labelled antibody (Ab). If malaria antigens are present these labelled antibodies bind to the malarial proteins to form labelled immune-complexes (as shown in the window on the iamge).



Part 2 The lysed red cell preparation containing free-labelled antibody and any labelled-immune-complexes then migrate along the strip (red arrow) pushed by the remaining buffer (blue arrow). If the test has been performed correctly the lysed red cells will migrate to the end of the strip. During this process they will first pass over (one or more) test lines (T) then a control control (C) (see below).



A POSITIVE" test result

If dye-labelled malaria-antigen/antibody complex is present then it will be "captured" by immobilised antibody that recognises the antibody/antigen complex, with the dye atteched to the antibody producing a visible line (a positive test line T). The remaining sample containing labelled antibody only continues to diffuse along the strip and is captured by a second antibody to form a control line (C). This control line simply indicates that test has been successfully performed.


For this simple test format detecting P.falciparum only, a positive result appears on the test as two visible lines (P.f and C). Shown below.



"NEGATIVE" test result

If dye-labelled malaria-antigen/antibody complex is not present then nothing will be captured by the malaria-specific test line (T). However, the sample containing labelled antibody that has not bound antigen will still bind to the immobilised antibody that forms control line (C) showing that the test has been successfully performed.



Again, for this simple test detecting P.falciparum only, this shows a negative result for P.falciparum antigens (absent P.f line), but a positive control line (C), confirms that the test was correctly performed. Shown below.