FROM OUR BLOG

False positive COVID test? by Dr Eleanor Galpin

Jul 23, 2021

You may have heard people talk about false positives with regards to COVID tests. What does this mean?

You may have heard people talk about false positives with regards to COVID tests. This does NOT mean that the lab has made a mistake, that the virus is actually not present.

A false positive result from a lab test would normally mean that something is reported to be present, but later is found not to be there at all. Not so with the COVID test.

The nasal swab tests are designed to detect a highly specific antigen protein, unique to SARS-Cov-2.

A laboratory PCR takes the few particles found in one’s nose and uses an enzyme to amplify them (to make exact duplicate copies) to many more of that protein. Then the sample is run through a test that looks for the SARS-COV-2 protein and reports whether or not it is detected.

This is a highly sensitive and specific test.

The problem is that the test cannot tell you if the viral particle/protein/antigen comes from a living or a dead virus! It cannot tell you if the virus is currently active or whether it is the remnants from a previous infection in the last three months!

If the test finds inactive viral debris, this is NOT a false positive. The viral antigen really is there. The lab has not made a mistake.

But what if the person has no symptoms at all?

Well, some people get totally asymptomatic COVID. In fact, many people do! Unfortunately, despite being asymptomatic or mildly symptomatic, they can still spread COVID.

And here is the answer to the problem. If I phone the lab and chat to a virologist, they can trace the test and tell me what the viral load is – in other words, how long it took for the test to become positive.

The longer it took for the machine to find the virus, the smaller the viral load and the more likely it is that this is not a current and active infection.

You can’t decide that yourself. Only the virologist can access the information.

Studies have shown that if it takes longer than 35 cycles to find the COVID protein, it is most likely an old infection that was active in the three months before the test was done. Many people are totally unaware that they were even sick. (They had mild symptoms and didn’t consider them relevant or worrying.)

With such a tiny viral load the person is not infectious. But a repeat test is usually done a few days later to confirm that the viral load is decreasing and not increasing! Therefore, caution should still be taken.

It is very important to remember that false NEGATIVES are very common.

The way the swab is taken, transported, handled, and processed can all lead to a person being declared negative when they are actually positive! Which is why, if you are sick, regardless of your test results, you should take precautions and stay away from vulnerable people!

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