Why Are People Testing Positive on Lateral Flow Tests, Then Negative on PCR?

There have been growing LFT and PCR test results discrepancies in the past few months. Several people in Southern England, UK, have reported having lateral flow test positive but PCR negative results.

Other reports indicate that some people have experienced a negative lateral flow test but symptoms that are very similar to those of Covid.

Scientists have put forward several explanations for these conflicting results, as we are going to find out in this article.

Do PCR and Antigen Tests Work Differently?

PCR and lateral flow tests (LFTs) work differently when it comes to screening for Covid-19.

PCR tests detect the presence of viral RNA. They are very sensitive, meaning they can detect even a small amount of the SARS-CoV-2 virus in a patient’s sample. This test is regarded as the gold standard for Covid-19 testing due to its sensitivity and accuracy.

On the other hand, lateral flow tests such as the Healgen rapid antigen test use lab SARS-CoV-2 antibodies to detect coronavirus antigens in a sample. Although they aren’t as sensitive as PCR tests, lateral flow tests are easy to use, and they produce results in minutes. LFTs or rapid antigen tests are also very accurate at detecting the coronavirus in asymptomatic people.

Notably, since LFTs aren’t as sensitive as PCR tests, they aren’t very effective at detecting the SARS-CoV-2 virus in a sample with a very low viral load. For instance, in the early stages of a Covid infection, a patient may not have a lot of virus in their nose or throat, although they may exhibit Covid-19 symptoms. This means if you take an LFT, you get a lateral flow test negative but still have symptoms of Covid.

Due to their low sensitivity, LFTs are more likely to give false negatives compared to PCR tests. This means that LFTs are best used to confirm the presence of a current infection, giving a person the all-clear. However, the test has good specificity, which means it’s unlikely to give a false positive result.

On the other hand, due to their high sensitivity, PCR tests can signal false positives. This often occurs in people who have already recovered from Covid-19 when the test detects a small amount of the virus in their bodies. 

What Test Is More Accurate

So, which is more accurate, the PCR test or lateral flow test?

Well, PCR and lateral flow tests do different things, which is why it’s difficult to compare their accuracy side by side.

While a FlowFlex rapid test and other LFTs will detect an active Covid-19 infection by picking up protein material from the surface of the virus, PCRs detect the SARS-CoV-2 virus by picking up the genetic material of the Covid-19 virus.

LFTs are often used to detect the virus in asymptomatic patients who are more likely to pass on the virus to others. On the other hand, PCRs are more effective at detecting the virus when it has been present in a person’s body for several weeks.

Nevertheless, PCR tests are known as the gold standard for Covid-19 testing because of their sensitivity. But despite LFTs having a bad reputation for low sensitivity, the test is very specific and helpful.

According to the UK Health Security Agency, LFTs have a specificity of 99.97%, meaning for every 10,000 LFTs taken by people who are Covid negative, only less than three people will get false positive results. This means that if your LFT test is positive, there is a high likelihood that you have a Covid-19 infection, as the test has a very low chance of giving a false positive.

Why Can People Be Positive on Lateral Flow and Negative on PCRs 

There have been reports of people in Southern England testing positive for Covid-19 with LFTs but negative on PCR tests. Others have reported lateral flow test negative but have symptoms of Covid-19. Potential explanations for the difference in these test results have been put forward by scientists, as explained below. 

For those who test negative on LFTs but still have Covid-19 symptoms, one explanation given is that the SARS-CoV-2 virus takes a few days to replicate to levels that can be detected by a rapid antigen test in the human body. Thus, if one is tested in the early stages of the infection, the test may not detect the virus, although the person will already have a Covid-19 infection.

One of the explanations given for the LFT and PCR test results discrepancies is that since the difference in results has been reported in one region, it could be due to a faulty batch of lateral flow devices or tests.

Another explanation given is that a new Covid-19 variant may be circulating that is yet to be detected by the PCR tests available.

There have been reports that children, too, have been faking positive LFT results by adding soft drinks to their samples, leading to a false positive LFT result and a negative follow-up PCR test.

Also, the increase in the positive LFTs and subsequent negative PCR test results coincide with the return of schools, leading to the belief that this could be due to the way school-going children are being tested.

There is also the potential that due to Covid-19 vaccination, the virus location and how it grows in the body may have changed, meaning that different swabs used for different tests may be capturing a lot less or more of the SARS-CoV-2 virus.

Alternatively, since both LFTs and PCR tests aren’t 100% reliable and accurate, the discrepancies in the test results may be due to how often each of the tests gives false negatives and positives.

The UK Health Security Agency has noted that it hasn’t experienced such result discrepancies on such a scale before and has promised to investigate the issue and hopefully give a good explanation of why this is happening.

Other Details You Should Know About Testing for Covid-19

It’s important to note that the UK government continues to encourage people to get tested and vaccinated to help in the fight against Covid-19.

The government has also provided new PCR test rules indicating that free PCR tests are no longer available to the public. However, free symptomatic LFT testing is still available to specific groups of people. Fully vaccinated UK travelers are also no longer required to take a PCR test on Day 2 of their arrival. During Day 2 of their quarantine, UK travelers are expected to take a mandatory LFT and a follow-up PCR test if the LFT is positive.

Further, the NHS advises people with Covid-19 symptoms such as high fever, continuous cough, and loss of smell and taste to self-isolate despite testing negative. Such people should avoid contact with other people, especially those with a weakened immune system, until they are no longer symptomatic. They should also wear a face covering and avoid crowded places if they must visit the outdoors.

Conclusion

In closing, reports of people testing positive on LFTs and negative on PCR tests have been reported on Twitter and other social media platforms. Most of these reports come from people living in Southern England. Although the UK Health Security Agency doesn’t have an explanation for these test discrepancies, it has promised to investigate the issue.