PCR is a test for the presence of DNA in body fluids. It can be used to detect and identify specific pathogens, such as HIV or hepatitis C. The lateral flow test is a rapid point-of-care diagnostic device that can detect certain bloodborne diseases, such as syphilis and hepatitis B and C.
The positive covid test then negative next day is a phenomenon that has been happening for years. It can be hard to diagnose, but it is important to know the signs and symptoms of the disease.
People who test positive on lateral flow tests but negative on PCR tests are becoming more common (Getty)
Christian Yates, University of Bath senior lecturer in mathematical biology
There have been an increasing number of complaints in the UK in recent weeks of individuals testing positive for Covid on a lateral flow test (LFT) but negative on a polymerase chain reaction (PCR) test. This is notable since we’ve been taught to think that PCR tests are the “gold standard” and LFTs are unsophisticated mass-testing devices, and that PCRs should detect instances missed by LFTs, not the other way around.
A variety of theories have been proposed. Some speculate that a defective batch of LFTs is causing individuals to test positive for Covid when they don’t have it. Others have speculated that a novel variation is spreading that is undetectable by conventional PCR tests. There have also been reports of youngsters utilizing the acidic qualities of soft drinks to fake a positive LFT test. A follow-up PCR test would reveal that these spoofers were negative.
The growth in these incidents is similarly closely timed with the resumption of schools and large increases in the number of cases among youngsters. It’s possible that the manner youngsters are assessed has anything to do with these positive-then-negative test sequences. Alternatively, vaccination may have altered where the virus develops best in the body, resulting in various swabbing methods catching more or less virus depending on the test type.
Given that neither test is 100 percent accurate, there may be a mathematical reason. It all comes down to the frequency with which these tests produce false positives and negatives.
Sensitivity and specificity
Despite its reputation for inaccuracy, the overwhelming probability is that you have Covid if you test positive on an LFT. Currently, upwards of 96 percent (7,000/7,297) of individuals testing positive on LFTs are genuine positives, as seen in the figure below.
This is due to the fact that LFTs are very’specific,’ meaning they don’t produce many false positives. There will be less than three false positives for every 10,000 LFTs done by truly Covid-negative individuals, according to Public Health England (formerly the UK Health Security Agency) (a specificity of 99.97 per cent).
However, they may add up. In a population of one million individuals, if 1% of the population has Covid, 297 people will have LFTs that indicate they have the virus when they don’t. And, since PCR tests are (nearly) 100% specific, they will very certainly receive a negative result if they follow up their LFT with one of these. This may help to explain some of the recent news.
Using typical values for test specificity and sensitivity, as well as a 1% community prevalence value, possible testing results for 1,000,000 individuals in the population. Kit Yates (Kit Yates)
However, this is just half of the equation when it comes to precision. LFTs have a poor reputation due to their low sensitivity, which means they produce a lot of false negatives. According to estimates, around 30% of the time when someone has Covid, an LFT will not detect it. PCR tests are much superior, with a false negative rate of just 5%.
However, a 5% false negative rate may result in a positive-then-negative testing cycle. With the present incidence of Covid, an LFT will accurately identify 7,000 of our one million individuals as having Covid, as shown in the figure above. The ‘confirmatory’ PCR test will result in an erroneous false negative for 5% of them — or 350 individuals.
It’s essential to note that, because to their great specificity, you may be quite certain that a positive LFT result is real – in our model, just 297 out of 7,000 are incorrect. Even if a negative PCR test confirms your positive result, it’s presently more probable that you have Covid than not (350 vs 297). If you’re symptomatic, your chances of being infected are much greater if you have a positive LFT but a negative PCR.
Unexpected outcomes attract greater attention.
There are a few more points to consider. The first is that around 3.5 percent of infected individuals test positive on an LFT but subsequently test negative on a PCR test (350 out of 10,000). Given the poor reputation of the LFT and the prominence of PCR testing as the “gold standard,” this may be greater than we would anticipate. This may explain why the frequency of reports of this positive-negative testing sequence is increasing.
When something unexpected occurs to us, we tend to attempt to make sense of it by discussing it with others. In recent days, reports of positive LFTs followed by negative PCR testing have flooded Twitter and made national headlines, prompting more individuals to come forward. It’s conceivable that this positive feedback loop is responsible for some of the rise in reports of this ostensibly uncommon occurrence.
Another thing to keep in mind is that how well (or badly) LFTs and PCRs perform in general and in comparison to each other is determined by the prevalence of Covid in the population. I was cautious in my calculations and estimated that one percent of individuals had Covid; nevertheless, the Office for National Statistics believes that it is now almost 1.5 percent of the population.
If this collapses, though, everything changes. The proportion of individuals who test positive on LFTs and are genuine positives will decrease, as will the number of positive LFTs followed by negative PCRs. If Covid becomes more common, the reverse will occur: we will witness even more of these’surprising’ testing sequences than we do now.
It’s essential to note that none of the ideas presented in this article have solid proof yet. However, determining if anything is really incorrect or whether this is simply a mathematical artefact has major implications for testing, contact tracing, and monitoring the UK’s present Covid issue.
Susan Hopkins, the UK Health Security Agency’s (UKHSA) chief medical advisor, has said that the agency is looking into the matter. The UKHSA admits that it doesn’t have an explanation yet, but it’s looking into it since it’s never seen anything like it before.
The UKHSA is hoped to be able to conduct a thorough inquiry and solve the riddle of the contradictory findings.
To read the original story on The Conversation, click here.
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The negative pcr test is a question that has been asked for a while. It’s not clear why people test positive on lateral flow then negative on PCR.
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