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Rapid COVID-19 antigen tests include a control line similar to this: Image of Covid Control line From Case Western Reserve rapid testing instruction available here.

What does the control bar/strip control for?

  1. All the chemicals required to test for COVID are present in acceptable concentrations. If there is COVID on the swab this test will show it.

  2. Other Indicators that should be present when the swab is correctly done have been identified. You have correctly swabbed and if you have COVID this test will show it.

  3. Both. This test is in good standing condition and you have properly swabbed. If you have COVID this will show it.

Since this test was distributed to the general public and there was little to no information about what the control bar/strip meant or did in the included documentation I was under the impression that option 3 was the most probable.

*All statements above assume no false positives or false negatives which I know can happen.

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Your explanation numbered 1 is correct.

The control line indicates that the antibodies required to detect the SARS-CoV-2 (the virus that causes COVID-19) particles have reached that location on the test strip.

Each product is slightly different, but the Rapid Response package insert notes:

Anti-SARS-CoV-2 antibodies are immobilized on the test region of the nitrocellulose membrane. Anti-SARS-CoV-2 antibodies conjugated to coloured particles are immobilized on the conjugated pad. A sample is added to the extraction buffer which is optimized to release the SARS-CoV-2 antigens from specimen. During testing, the extracted antigens bind to anti-SARS-CoV-2 antibodies conjugated to coloured particles. As the specimen migrates along the strip by capillary action and interacts with reagents on the membrane, the complex will be captured by the anti-SARS-CoV-2 antibodies at the test region. Excess coloured particles are captured at the internal control zone.

This image I made for Wikipedia many years ago explains how this works: enter image description here

In this case, the SARS-CoV-2 nucleoprotein antigens are the green dots. Antibodies specific for the nucleoprotein antigens are attached to colorful gold dots. When you place the a sample with virus on the test strip, the antibodies with the gold bind to the virus. As it moves up the strip by capillary action, another part of the virus binds to the antibodies pictured here in black. The increased local concentration of gold particles is visible on the strip.

Antibodies with gold attached that are not captured by the black region reach the blue region, where the antibodies themselves are bound. Thus, the control line only ensures that antibodies from the space where you drop the test solution have reached a location further away than the SARS-CoV-2 specific region. It does not ensure any particular composition of the sample.

The package insert describes this:

A coloured band at the control region serves as a procedural control, indicating that the proper volume of specimen has been added and membrane wicking is working.

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    I always assumed that the control line asserts that the swab has been used correctly (I thought it would test for something that is always present in a human nose). Do I understand it correctly that if I would skip the part where the swab goes into my nose, the control line would still show? Would the control line even show if I just put some drops of water into the test?
    – Lukas Boersma
    Jun 1, 2022 at 11:04
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    @LukasBoersma probably yes and yes. This is also an experiment you can easily do in practise-
    – lalala
    Jun 1, 2022 at 11:12
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    @lalala I did that (didn't swab but simply rinsed the virgin cotton end in the solution and put the required number of drops in the test receptacle) and lo and behold, the control bar lit up. It's just a primitive marker to show that the fluid reached it, coming from the receptacle area. I don't understand why they don't put antibodies against some common human DNA there, so that an insufficient (dry, not enough material sampled, or not rinsed properly) swab would not light up the control. Perhaps too costly or too unreliable. Perhaps Ian knows why?
    – Peter - Reinstate Monica
    Jun 1, 2022 at 17:10
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    @Peter-ReinstateMonica I love a good practical experiment. Like you, I suspect cost is the reason. Back when tests were actually rigorously approved by a regulatory agency, you'd need to get each additional antibody systematically tested and quality controlled. Plus, there's the actual cost of producing and isolating it and conjugating it to gold or some other chemophore.
    – Ian Campbell ♦
    Jun 1, 2022 at 17:14
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    @IanCampbell Probably the regulatory effort. The standard reagent they have now is true and tested and mass produced (presumably). Producing and isolating and conjugating a human "anti"gen detector as such may not be much different from the stuff they use now. In fact, they'd save the antibodies in the receptacle area (your black things with red dots).
    – Peter - Reinstate Monica
    Jun 1, 2022 at 17:22

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