As Omicron increases the number of Covid-19 cases worldwide, concerns are expressed about whether lateral flow and polymerase chain reaction tests are able to detect the highly infectious variant.
Lateral flow testing (LFT), also known as rapid antigenic testing, has never been the gold standard for coronavirus testing.
But because they are cheap and give results in about 30 minutes, they are widely used, especially for home testing.
Polymerase chain reaction (PCR) tests are performed in the laboratory and, although more expensive, are generally more sensitive and are less likely to give a false negative – when an infected person is told that they are. not.
But there is more to it. Here we take a look at what can happen with the tests and Omicron.
What are the concerns about LFTs and Omicron?
The media have reported cases of people repeatedly testing negative on FTL despite showing symptoms of Covid-19, only to later test positive on a more sensitive PCR test.
Lateral flow tests use antibodies to detect viral proteins (antigens) in the nasal passages of infected people. Their sensitivity ranges from about 40 to 60 percent.
PCR tests look for the genetic material of the virus and involve several cycles of heating and cooling in a machine in which this genetic material is replicated, making the tests more sensitive.
Because of this difference in sensitivity, the fact that some people have recently tested negative multiple times with LFTs, only to then test positive with a PCR test, is not in itself evidence that LFTs have a harder time. detect Omicron than other variants.
“They are less sensitive. Someone is expected to do a number of negative lateral fluxes and then have a positive PCR, ”said Paul Hunter, professor of medicine and infectious disease specialist at the University of East Anglia in the UK. United.
What are the people in charge of LFT and Omicron saying?
In a December 17 briefing document, the UK Health Security Agency said “initial data” indicated that LFTs used by the UK’s testing and traceability service were detecting Omicron.
These tests use two or more different antibodies that recognize distinct parts of the nucleocapsid, which is the protein coat that surrounds the genetic material of the pathogen.
According to the UKHSA document, the Omicron nucleocapsid exhibits “a single mutational difference from other Sars-CoV-2 variants”.
“As such, the general risk for LFD [lateral flow device] the performance affected by Omicron was considered poor, ”the organization said, adding that further laboratory analyzes were underway.
An article published by South African researchers just before Christmas indicated that with Omicron, oral swabs are better than nasal swabs at detecting coronavirus. With Delta, it’s the opposite.
As a result, they said that as Omicron becomes dominant, it may be necessary to re-evaluate the tests used. Their article has yet to be peer reviewed.
Additionally, in a statement posted online this week, the United States Food and Drug Administration (FDA) said rapid tests may have lower sensitivity with Omicron.
“The first data suggests that antigen tests detect the Omicron variant, but may have reduced sensitivity,” the FDA said.
Why does Omicron appear less often on LFTs?
Dr Michael Mina, scientific director of eMed and former associate professor at Harvard University, offered a possible explanation for why people with symptoms may have repeated negative FTLs.
Writing online in mid-December, he suggested that now that many people have been vaccinated or have had a Covid-19 infection, their immune systems work quickly when infected.
This causes symptoms to develop early, even before viral levels are high enough to be detected by LFT. Whether this actually happens, however, is not confirmed.
Another suggestion – also unconfirmed – is that the Omicron variant is transmitted and can cause infections, even when the virus levels in the nasal passages are low and difficult to detect for LFT.
If so, that would mean that it is even more crucial for a person to self-isolate when exhibiting symptoms of Covid-19 even if their LFT is negative, as they could be infectious.
How Much Should We Trust in Lateral Flow Tests?
Dr Al Edwards, associate professor at the University of Reading School of Pharmacy in the UK, said LFTs have consistently missed some positive cases.
“There may be some differences with the ability of lateral flow tests to detect variants, but they have never been able to detect all cases,” he said.
They are, he said, “very, very reliable” as a red flag – meaning if you test positive you have the virus and could infect others – but have never been reliable. as a green flag because they can’t confirm that you do. do not have the virus and are not contagious.
“It was advised to use them before meeting people,” he said. “You should use them, but they can never prove you’re safe. “
If a person is considering meeting a friend who is not likely to be vulnerable to Covid-19, he has indicated that it may be a good idea to do an LFT first and continue the meeting if the result is negative.
On the flip side, if a person is considering seeing a more medically vulnerable person, such as an elderly parent, they may not want to rely on LFT in the same way.
Professor Hunter said people with a negative LFT result were “probably not so contagious” because the virus levels were likely lower.
“Even if they have the virus, they probably don’t shed a lot, so they’re probably not infectious,” Dr Edwards said.
Can Omicron escape nasal swab PCR testing?
LFTs are not the only tests whose effectiveness in some cases appears to be affected by the emergence of Omicron.
While the WHO said at the end of November that “widely used PCR tests continue to detect infection, including infection with Omicron,” a subsequent analysis found exceptions.
The FDA has identified some tests that may not identify a positive case when the person is infected with Omicron. Indeed, the part of the genetic material of the virus sought by these particular tests has changed in the new variant.
In an online update released this week, the FDA pointed out that a PCR nasal swab test called Revogene Sars-CoV-2, produced by Meridian Bioscience, as “should not detect Omicron.”
This is because the test has a single target in the genetic material of the coronavirus, but part of that target has been removed in Omicron. The FDA recommends that you do not use this test until the problem is resolved. The FDA guidelines relate specifically to the tests for which it has granted authorization.
Another test, produced by Tide Laboratories, had the same problem, but changes so that there is more than one genetic target means it should now detect Omicron.
Update: January 1, 2022, 7:00 a.m.