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If someone does a Covid lateral flow test and the control line is not visible, the test is invalid regardless of the visibility of the test line. A missing control line could be due to insufficient buffer volume or damage to the membrane coating in the area where the control line should appear. In either case, the visible test line would be true-positive. Is there a scenario where the test line is visible, but the control line is not visible, and the sample is Covid-negative? If so, how does this work on the molecular level?

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    $\begingroup$ I’m voting to close this question because it is a question about the interpretation of a clinical test, and not about a problem in biology. It may be appropriate for our sister site, SE Medical Sciences $\endgroup$
    – David
    Dec 16, 2023 at 21:53
  • $\begingroup$ @David I'm not asking for the clinical interpretation of the test. This is something I already know. I am asking about how such a result can occur on the molecular/technical level. $\endgroup$
    – AnjaM
    Dec 17, 2023 at 8:17
  • $\begingroup$ Indeed, but that is neither a problem in biology or a in a laboratory research technique. It is about the operational details of proprietary clinical tests, which falls within the remit of the newer SE site I mentioned. Please move it there. $\endgroup$
    – David
    Dec 17, 2023 at 20:41

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If someone does a Covid lateral flow test and the control line is not visible, the test is invalid regardless of the visibility of the test line.

Your first sentence is your entire answer. If there's no control line, one or more of many possible issues have occurred, making that test's results null and void. The test should be rerun using fresh reagents 100% of the time. That's it. Without control, you cannot trust the appearance (or lack thereof) of the test line.

The same is true in laboratory settings. If I'm running an assay and my control doesn't work, I toss the entire thing and start over.


As far as how this could happen, as I said there are many possibilities, some of which could have influenced others. Typically the tests are arranged so that the buffer reaches the test area first, then the control area. As you say, lack of a control line could indicate that there was not enough buffer present to successfully diffuse the detection reagents to the control area.

However, you claim that the sample is supposedly "COVID-negative". I would first question how that conclusion was arrived at. PCR testing is the gold standard, and viral load can change relatively quickly in the body, so unless both types of tests were performed at the same time, you can't draw any conclusions about presence or absence of detectable levels of SARS-CoV-2 in the lateral flow test.

Let's assume, for the sake of argument, that the sample truly is negative for SARS-CoV-2. The most likely cause of this test pattern is some sort of manufacturing deviation, most likely several that worked together. Multiple possibilities come to mind - incorrectly formulated reagents, reagents being applied to incorrect areas of the strip, damage to the strip itself, possibly leading to the removal of the detection reagents, and so on. Most if not all eventually lead to human error of some sort, whether it is imperfect quality control, lack of verification/validation of all manufacturing steps, machines not working properly due to lack of maintenance, etc. An awful lot of complexity goes into producing these tests, and there are many steps where errors can occur. Without having access to the test strip in question and the manufacturing facility itself, it is impossible to definitively identify the cause(s) of the erroneous result.

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  • $\begingroup$ I'm aware of how it should be interpreted. I'm interested in how this (positive test line, negative control line) can happen on the molecular level of the test, and whether/how this can happen if the sample is Covid-negative. $\endgroup$
    – AnjaM
    Dec 17, 2023 at 8:19
  • $\begingroup$ @AnjaM my apologies. Please see the edited answer above. $\endgroup$
    – MattDMo
    Dec 17, 2023 at 19:45
  • $\begingroup$ Thanks, this answers my question! As a context: The sample was indeed Covid-positive, as confirmed by another test. I wondered about the theory behind this since the manufacturer claims that a T-line without a C-line is not a valid test, but I couldn't think of a situation how a sample with a T-line would not be positive (except for if additional errors happened, like you listed - but they wouldn't influence the C-line). So in the case of T-line positive, it seems to me that chances of the sample being positive are the same independently of the C-line being visible or not. $\endgroup$
    – AnjaM
    Dec 18, 2023 at 8:14

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