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Clearly the test line binds some component of the coronavirus, I believe most commonly the nucleocapsid protein (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8049777/). However, despite extensive googling (and Google is doing a lousy job of understanding my question) I have not been able to figure out what is being bound by the control line. Probably it differs by test manufacturer, but I'm sure there are some relatively common antibodies that are used in the control line?

My best guess is that it should test for some common mucin or collagen or elastin but this is based on nothing.

The reason I'm asking is that I want to understand if the "control" line being present is a good indicator of the test having been done properly. Let's say some person:

  • rubs the swab on a different body part than the nasal mucosa
  • or doesn't touch the body at all with it
  • or swabs the nasal mucosa very briefly
  • or only inserts the swab into the buffer solution extremely briefly

would the control line still show up?

I'm not simply trying this myself at home because it's annoying and expensive to get lots of tests simply to waste them to test their limits.

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According to biotrend (SARS-CoV-2 (Covid-19): Rapid Antigen Test for Diagnosis), the control line has an immobilised Goat anti-chicken antibody. There, a chicken-IgY gets stuck which was carried from a conjugate pad by the sample fluid.

So even pure buffer solution would yield a positive signal at the control line. It seems that the control only tests for proper reactivity of the antibodies contained in the conjugate pad, and not for proper usage by the user:

A colored band in the control region serves as a procedural control, indicating that the appropriate sample volume has been added and the membrane is functioning.

This manual (health.govt.nz) reports that whole blood and Mucin do not interfere with the test (including control?). It sounds as if biological fluids are meant to be ignored by the test in general.

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    $\begingroup$ @user21820 No, this is not stupid because you need a control that shows that your test is valid which is independent of the sample. Having a positive control would sometimes be nice but makes the interpretation more complicated. Not to speak of the need to have a control that under no circumstances will produce a false positive result. $\endgroup$
    – Chris ♦
    Sep 30, 2022 at 5:50
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    $\begingroup$ @Chris: You claim "you need". I disagree. What we need from the test is a way to distinguish between covid infected and non-infected. The way the tests have been designed is inferior for that purpose. For what I suggested, there is no complicated interpretation at all. (1) No control line ⇒ Test invalid. (2) Control line ∧ Test line ⇒ Infected. (3) Control line ∧ No test line ⇒ Likely uninfected. $\endgroup$
    – user21820
    Sep 30, 2022 at 5:56
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    $\begingroup$ @user21820 Believe me, you need this. What do you do with tests where the positive control is absent but the test band is present? Also: How do you control if the test is working at all? Antibodies are stable, but still storing conditions are important to ensure a result. $\endgroup$
    – Chris ♦
    Sep 30, 2022 at 6:20
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    $\begingroup$ @markur Test line obviously means you have COVID. Control line missing obviously indicates you did something wrong with the sample. Intuitive conclusion: you did something wrong, but COVID was detected anyway; if that bothers you, take another test. $\endgroup$
    – user253751
    Sep 30, 2022 at 22:59
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    $\begingroup$ @markur: This isn't just about malicious testing. Even an honest person can't be sure that they have swabbed enough if the control line doesn't give them any feedback. $\endgroup$
    – user21820
    Oct 1, 2022 at 8:54

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