French News: Why do some COVID tests give false results and how often are “false positives”? – why some covid tests give false results and how common is a “false positive”

(Adrian Esterman, University of South Australia)

Adelaide, Australia, June 6 (The Conversation) Two cases of COVID-19, previously linked to the current coronavirus outbreak in Melbourne, are now falsely classified as positive (infected) cases.

These cases are not included in Victoria’s official figures, while many risky sites linked to these cases have also been removed.

The main and “gold standard” test to identify the SARS-CoV-2 virus responsible for COVID-19 is the reverse transcriptase polymerase chain reaction (RT-PCR) test.

The RT-PCR probe is highly specific. This means that if someone is not really infected, there is a good chance that the test results will be negative. This test is also very sensitive. So if someone is truly infected with the virus, there is a good chance that the test result will be positive.

But even though the test is very specific, there is a slight apprehension that even if a person does not have the infection, they will still be positive in the test result. This is called a “false positive”.

To understand this, you first need to know how the RT-PCR test works. Most people heard of the PCR test during Kovid’s day, but how it worked remains a mystery.

To try to understand it in simple and short terms, chemicals are used to extract RNA (ribonucleic acid, a type of genetic material) from samples taken from the nose or throat. This includes a person’s normal RNA and if the SARS-CoV-2 virus is present, then their RNA.

This RNA is then converted into DNA (deoxyribonucleic acid) – this is called “reverse transcriptase (RT)”. Short segments of DNA are amplified to detect the virus. Using a special type of fluorescent dye, a test is identified as negative or positive depending on the brightness of the light after 35 addition cycles or more.

The main reason that false positive results show up is a lab error and an off-target reaction i.e. the test cross-reacted with something that is not SARS-CoV-2 .

Lab errors include clerical errors, testing the wrong sample, contamination of another sample with someone else’s positive sample, or problems with the reagents used (such as chemicals, enzymes and dyes). Those who have had Kovid-19 and have recovered, sometimes they also appear infected on examination.

To understand how common these false results are, we need to look at the false positive rate, which is the ratio of people tested and found positive even though they are not infected.

Authors of a recent prepublication (an article that has not been independently reviewed or validated by other researchers) reviewed the evidence on false positive rates for RT-PCR testing.

They combined the results of several studies and found the rate to be 0-16.7%. In 50 percent of these studies, this rate ranged from 0.8 to 4.0 percent.

A systematic review of false negative rates in RT-PCR tests found that the false negative rate was 1.8 to 5.8 percent. However, the review acknowledged that the quality of most of the studies was poor.

According to the author of this article, no test is perfect. For example, if the rate of false positives in the RT-PCR test is assumed to be four percent, then for every 1,000 people found negative in the test and who do not have a true infection, 4,000 will be false positive. The problem is, we’ll never get to know most of them. The person found infected will be asked to live in isolation and anyone who comes in contact with them will assume they have an illness without symptoms.

Anyone who is said to be infected because of a bad test is forced to live in isolation. It is very stressful to tell someone that you are suffering from a fatal disease, especially for the elderly, as their health is already fraught with risk.

Likewise, false negative results are obviously of great concern as it can be dangerous for infected people to roam freely in the community.

Overall, it must be said that false negative or false positive results are going to cause problems.

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