Why are some COVID test results false positives and how frequent are they?

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There are a few reasons why an RT-PCR test can result in a false positive. Credit: Shutterstock

There have now been two COVID-19 cases previously related to the current Melbourne outbreak reclassified as false positives.

They are no longer included in the official Victoria case counts, while several exhibition sites linked to these cases have been removed.

The main and “golden pattern” to detect SARS-CoV-2, the virus that causes COVID-19, is the reverse transcriptase polymerase chain reaction (RT-PCR) test.

The RT-PCR test is very specific. That is, if someone does not really have the infection, there is a high probability that the test will be negative. The test is also highly sensitive. Therefore, if someone is actually infected with the virus, there is a high probability that the test will return positive.

But while the test is very specific, this still leaves a small chance that someone who does not have the infection will get a positive result. This is what is meant by “. ”

First, how does the RT-PCR test work?

Although in the COVID era most people have heard of the PCR test, how it works it is understandable a bit of a mystery.

In summary, after removing a swab from the nose and throat, chemicals are used to extract the RNA (ribunocleic acid, a type of genetic material) from the sample. It includes the usual RNA and RNA of a person from the SARS-CoV-2 virus, if any.

This RNA is then converted to deoxyribonucleic acid (DNA), which is what the “reverse transcriptase” bit means. To detect the virus, tiny segments of DNA are amplified. With the help of some special fluorescent dye, a sample is identified as positive or negative depending on the brightness of the fluorescence after 35 or more amplification cycles.

What causes false positive results?

He main reasons because false positive results are laboratory error and off-target reaction (i.e., the test reacts crosswise with something other than SARS-CoV-2).

Laboratory errors include clerical errors, testing for an incorrect sample, cross-contamination by a positive sample from another person, or problems with the reagents used (such as chemicals, enzymes, and dyes). Someone who has had COVID-19 and has recovered may also show a false positive result.

How common are false positive results?

To understand how often false positives occur, we look at the false positive rate: the proportion of people tested who do not have the infection but present a positive test.

The authors of a recent publication prepress (a paper that has not yet been peer-reviewed, nor independently verified by other researchers) conducted a review of evidence on false-positive rates for the RT-PCR test used to detect SARS-CoV-2.

They combined the results of multiple studies (some specifically examined PCR testing for SARS-CoV-2 and others examined PCR testing for other RNA viruses). They found false positive rates of 0 to 16.7%, with 50% of studies ranging from 0.8 to 4.0%.

The false positive indices in the they were mainly based on quality assurance tests in laboratories. In real-world situations, accuracy is likely to be lower than in laboratory studies.

A systematic review looking at the false-negative indices in the RT-PCR tests for SARS-CoV-2, the false-negative indices were found to be 1.8 to 58%. However, they point out that many of the studies were of poor quality, and these findings are based on low-quality evidence.

No test is perfect

Let’s say, for example, that the real-world false-positive rate is 4% for SARS-CoV-2 RT-PCR testing.

For every 100,000 people who test negative and don’t really have the infection, we would expect to have 4,000 false positives. The problem is that for most of these we never know. The person who has tested positive is asked to quarantine and everyone assumes that they have asymptomatic illness.

This is also confused with the fact that the rate of false positives depends on the underlying prevalence of the disease. With a very low prevalence, as we see in Australia, the number of false positives can end up being much higher than the actual number of positive positives, known as false positive paradox.

Due to the nature of Victoria’s current outbreak, authorities are likely to be very alert to test results, which could lead to false positives being collected. He Said the Victorian government: “After analysis by an expert review group and re-testing through the Victorian Infectious Disease Reference Laboratory, two cases related to this outbreak have been declared false positives.”

This does not make it clear if both people were retested or if only the samples were retested.

Either way, it’s unlucky to have two false positives. But given the large number of people who get tested every day in Victoria, and the fact that we know false positives will occur, it’s not unexpected.

The broader implications

For a person who received a false positive test result, they would be forced to go into quarantine when it was not necessary. Being told you have a potentially lethal disease is very stressful, especially for the elderly or at risk for other health conditions. They would probably also worry about infecting other members of their family and could lose their jobs while in quarantine.

Particularly given authorities initially pointed to these two cases as examples transmission of the virus through a “passenger” contact, no doubt many people have wondered if without these cases, Victoria could not be closed. This is just a conjecture and we cannot know one way or the other.

False negative results are clearly very worrisome, as we do not want infectious people to go around the community. But false positives can also be problematic.


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Citation: Why are some COVID test results false positives and how often are they? (2021, June 4), retrieved June 4, 2021 from https://medicalxpress.com/news/2021-06-covid-results-false-positives-common.html

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