How Reliable Are COVID-19 Rapid Tests for Detecting Omicrons?

The good news: Rapid antigen tests can definitively detect omicrons. The bad news: Some research indicates that the rapid test may be less sensitive to omicrons than other forms.

This means the tests can give more false negatives, especially in the early days of infection. And preliminary data from a small study suggests that people may be infectious before receiving a positive result on a rapid test.

You may not be able to count on a negative rapid-test result in the early days of infection. Repeated testing over a few days may be necessary to get accurate results. If you’re testing before an indoor gathering, you’re still taking a risk.

Abbott Laboratories, which makes the popular BinaxNow at-home rapid test, said that extensive testing showed that “BinaxNow continues to detect Omicron variants at comparable viral load levels to all other variants and the original SARS-CoV-2 strain.” keeps.” The company also pointed to a recent preprint study from Brigham and Women’s Hospital and Harvard, which showed BinexNow’s strong performance with Omicron.

Quiddell Corp., the maker of the Quickview test, said it tested using live Omicron virus from South African samples that confirmed the test could detect the variant.

So how should you understand conflicting information—and what should you do about it?

What does research say

Preliminary data is mixed. The research has looked at different trials in different countries, making it difficult to draw firm conclusions.

A recent preprint, which has not been peer-reviewed, examined the performance of antigen tests with cultured virus samples from oomicron-infected people and those with other variants. The study found that the tests were less sensitive at detecting omicrons. In a separate study in the Journal of Clinical Microbiology, Australian researchers conducted 10 antigen tests with Omicron as well as with Delta.

The US Food and Drug Administration said in late December that “early data suggests antigen tests detect the Omicron variant, but sensitivity may be low.” This statement was based on unpublished laboratory data from research at Emory University supported by the National Institutes of Health. ,

The study compared the performance of the rapid test on live virus samples containing the Omicron variant against samples containing the delta variant. The researchers found that when they diluted the Omicron and Delta samples, the rapid tests were better able to detect the virus in the Delta samples, says Bruce Tromberg, director of the NIH’s National Institute of Biomedical Imaging and Bioengineering, which supported the Emory study. is funding.

If you’re feeling sick but repeatedly getting negative rapid-test results, a separate recent small study conducted in a real-world setting during an omicron surge may help explain why. It was found that after identification of a positive case in the PCR test, it took an average of three days to detect the infection in the rapid test.

The study included data from 30 people who took daily saliva-based PCR tests and rapid nasal antigen tests in December at five workplaces in New York and San Francisco. All workers were vaccinated; Most were promoted.

The study found that rapid tests produced false negatives for two days after a positive PCR test, even though most people had high levels of the virus to infect other people. At least four people passed the virus to others over the course of three days to give a positive result on the rapid test.

With Omicron, people were very contagious before a rapid test returned a positive result, says Blyth Adamson, lead author of the study and founder and CEO of Infectious Economics, a public-health consultant. Dr. Adamson is also an employee at Flatiron Health, an independent associate of Roche. Another Roche unit conducts PCR tests. One of the paper’s co-authors is the CEO of the company that provided the saliva-based COVID-19 test used in the study.

The scientists not involved in the study noted that it is possible that positive cases in PCR tests show up sooner than rapid tests because PCR tests were done with saliva, while rapid tests took nasal samples. Preliminary data from other studies has indicated that omicrons may appear earlier in saliva tests than in nasal swabs.

In general, rapid tests have a lower sensitivity than PCR tests, meaning they produce more false negatives. The FDA authorizes tests provided they have 80% sensitivity – so that they produce a false negative 20% of the time. Most tests on the market have between 85% and 95% sensitivity.

Why Rapid Tests With Omicron May Be Less Sensitive

Many people have some level of protection when Omicron is taking a hit as a result of natural infection or vaccination. People who have been vaccinated and boosted have peak concentrations of the virus — when rapid testing is most likely to detect it — three times the risk of infection, says Omei Garner, director of clinical microbiology for UCLA Health System. Or maybe not until four days later. He says the delay could result in more false negatives during the first few days.

Another possible explanation: Because it appears that people with lower levels of omicrons can spread the virus before they have high enough levels to be detected in rapid testing, people may be infectious, scientists say. have to say.

Dr. Adamson says the incubation period of an omicron—the time between when you’re exposed to the virus and when you become infectious—seems to be as short as 12 to 24 hours.

This means someone could get a negative rapid-test result and be contagious hours later, says Gigi Gronwall, a senior scholar at the Johns Hopkins Center for Health Security.

Tests also seem to be in the wrong place. A sore throat is a common first symptom of omicron infection, and some scientists speculate that a sore throat may be the detection of the virus earlier than nasal inflammation. FDA advises against throat swabs, but the practice is endorsed by the UK

How should you use the rapid test?

Rapid antigen tests are a useful tool if we are smart about how we use them.

Because the data suggests that rapid tests early in the Omicron infection may give more false negatives, you have a better chance of getting accurate results if you wait a day or two after you develop symptoms for the test. Will, says Caitlin Jetelina, assistant professor of epidemiology at the University of Texas Health Science Center in Houston. If your test is negative, do a second test a day or two later, or get a PCR test if you can. (Easier said than done these days.)

If you’re considering using them to screen people before they gather, understand the limitations of rapid tests. Rapid tests before a wedding or other large indoor gathering can rule out early infections that may be contagious, Dr. says Adamson.

If you have a positive rapid test, you almost certainly have COVID-19, says Dr. Gronwal. False positives are rare, especially when the case rate is as high as it is now. You do not need to confirm a positive rapid-test result with a follow-up PCR test unless an employer or other institution requires it.