Two COVID-19 cases previously don’t link to Melbourne’s current outbreak have now been reclassified as false positives.
While a number of exposure sites linked to these cases have been removed, they are no longer included in Victoria’s official case counts. The reverse transcriptase-polymerase chain reaction (RT-PCR) test is the main and “gold standard” test for detecting SARS-CoV-2, the virus that causes COVID-19.
The RT-PCR test is highly specific. If someone truly has the infection there is a high probability the test will come out negative in the RT-PCR test. However, the test is highly sensitive and if someone truly infected with the virus has a high probability that the test will come positive.
But even though the test is highly specific, that still leaves a small chance someone who does not have the infection returns a positive test result. This is what’s meant by a “false positive”.
First off, how does the RT-PCR test work?
How does RT-PCR work is understandably a bit of a mystery, although in the age of COVID-19 most people have heard of the PCR test?
After a swab has been taken from the nose and throat, chemicals are used to extract the RNA (ribunocleic acid, a type of genetic material) from the sample. This contains a person’s usual RNA and RNA from the SARS-CoV-2 virus, if present. To detect the virus, the tiny segments of the DNA are amplified.
The RNA id then converted to deoxyribonucleic acid (DNA) — this is what the “reverse transcriptase” bit means. A sample with the help of some special fluorescent is identified to be positive or negative based on the brightness of the fluorescence after 35 or more cycles of amplification
What causes false-positive results?
The major reasons for false-positive results are the errors made in laboratory and off-target reaction (that is, the test cross-reacting with something that’s not SARS-CoV-2).
Laboratory errors include clerical error, testing the wrong sample, cross-contamination from someone else’s positive sample, or problems with the reagents used (such as chemicals, enzymes, and dyes). Someone who recovered from Covid-19 or who had Covid-19 shows 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 return a positive test.
According to authors of a recent preprint (a paper that hasn’t yet been peer-reviewed, or independently verified by other researchers) undertook a review of the evidence on false-positive rates for the RT-PCR test used to detect SARS-CoV-2. They combined the results of multiple studies (some looked at PCR testing for SARS-CoV-2 specifically, and some looked at PCR testing for other RNA viruses). In the study, they found false-positive rates of 0-16.7 per cent, with 50 percent of the studies at 0.8-4.0 percent.
The false-positive rates in the systematic review were mainly based on quality assurance testing in laboratories. It’s likely that in real-world situations, accuracy is poorer than in laboratory studies. A systematic review looking at false negative rates in RT-PCR testing for SARS-CoV-2 found false-negative rates were 1.8-58 percent.
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, the real-world false positive rate is 4% for SARS-CoV-2 RT-PCR testing. For every 100,000 people who test negative and truly don’t have the infection, we would expect to have 4,000 false positives.
The problem is that for most of these we never know about them. The person who tested positive is asked to quarantine, and everyone assumes they had the asymptomatic disease. This is also confounded by the fact the false positive rate is dependent upon the underlying prevalence of the disease.
With very low prevalence as we see in Australia, the number of false positives can end up being much higher than the actual true number of positives, something is known as the false-positive paradox. Because of the nature of Victoria’s current outbreak, authorities are likely to be extra vigilant with test results, potentially making it more likely for false positives to be picked up.
The Victorian government said: “The following analysis by an expert review panel, and retesting through the Victorian Infectious Diseases Reference Laboratory, two cases linked to this outbreak have been declared false positives.” This doesn’t make clear whether the two people were retested, or just the samples were retested. Either way, it is unlucky to have two false positives. But given the large numbers of people being tested every day in Victoria at present, and the fact we know false positives will occur, it is not unexpected.”
The broader implications for an individual who received a false-positive test result, they would be forced to go into quarantine when there was no need. Being told you have a potentially lethal disease is very stressful, especially for elderly people or those at risk because of other health conditions.