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I have a COVID home test kit which produces C and T (control and test) stripes when the solution is applied to the strip. Something similar happens in pregnancy test kits.

I understand the purpose of a control and test response, but what is the difference in mechanism?

In a negative test, why will C show up even though T does not?

In a failed test, why will C not show up even though T does?

Also, these stripes are somewhat distant from the spot at which the solution is applied. Why the separation?

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    $\begingroup$ @tyersome Laboratory techniques in biology are on-topic here, and though I think there is an argument that it would fit more securely at Medical Sciences it's reasonable to have it here as well. $\endgroup$
    – Bryan Krause ♦
    Jul 22, 2021 at 22:24
  • $\begingroup$ @BryanKrause — OK, I'll withdraw my close vote. $\endgroup$
    – tyersome
    Jul 23, 2021 at 18:45

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This is a lateral flow assay.

enter image description here [image source]

A sample is applied at one end of the strip and flows across to the other side by capillary action. It first encounters antibodies against the target antigen and which are conjugated to some reporter (in the image above, the reporter is colloidal gold). When the sample contacts these antibodies, they too will begin to move across the test strip. If the target antigen is present in the sample, in your case COVID, these antibodies will bind to it.

The sample then crosses the test line. The test line contains antibodies, immobilized to the strip, against the target antigen. If the sample contains the target antigen, it will bind to these antibodies and stop moving. Since the antigen is also bound to the reporter antibodies encountered earlier, these antibody/reporter conjugates will become concentrated at the test line and produce a visual change on the strip.

Finally the sample encounters the control line. The control line contains antibodies against the reporter antibodies. Regardless of whether the target antigen is found in the sample, the reporter antibodies will bind here and produce a visual line. The purpose of the control line is to give a visual indication that the sample has in fact travelled across the whole strip.

In a negative test, the lack of antigen means the reporter antibodies will not stop moving at the test line and will only be bound at the control line.

There are several possible reasons for a failed test (when no lines are visible or the test line is visible and the control line is not). Among these, it’s possible the sample has simply stopped migrating across the strip before reaching the test and/or control lines. Or it could be that the antibodies have degraded due to improper or prolonged storage such that they no longer bind properly.

Note that this answer is just a general description of a sandwich lateral flow assay. There can be variations of this procedure in specific tests.

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    $\begingroup$ There's another possible explanation for test failure - antibodies failed to bind. This could be a result of improper storage, with too hot most likely, followed by degradation of the antibodies (time). $\endgroup$
    – bob1
    Jul 22, 2021 at 20:26
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    $\begingroup$ @bob1 That’s a great point, thanks. I’ve updated the answer accordingly. Feel free to edit further. $\endgroup$
    – canadianer
    Jul 22, 2021 at 22:05

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