When to trust your covid test results and when to question them

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After dodging Covid-19 for nearly 2.5 years, I knew my time would come. But after all these conflicting test results, I was puzzled.

How to explain the wobbly results? I asked doctors, academics and scientists and this is what they said:

First, test deviations seem to be increasingly common with Omicron and its sub-variants, so a little common sense helps. If you are like me and you live with people who have Covid-19 and you feel symptoms developing, you are probably developing Covid-19. So even if your tests say otherwise, stay home.

For rapid antigen testing, serial testing is the name of the game. Test daily or every other day for up to a week if you can. Once you get a positive result, you can trust it, even if it’s a weak line.

In general, PCR tests are more sensitive than rapid antigen tests and will detect an infection sooner. But even PCR tests can have false negatives – although the rate is less than 1% – doctors and scientists say.

My negative PCR test seems to belong to this small group. I noticed that the test site I was using was not as comprehensive. They certainly didn’t count the 15 seconds per nostril I did at home and barely pushed the swab up my nose.

They could also have missed a very faint line if they hadn’t watched carefully.

“Anything you ask humans to do will have some amount of error,” says Gigi Gronvall, senior researcher at the Johns Hopkins Center for Health Security.

But it’s also possible that I experienced rapid viral growth, some doctors and scientists said. So, while my morning swab was negative, the virus may have rapidly proliferated in my nose, resulting in my faint positive line eight hours later.

“Viruses grow rapidly,” says Dr Gronvall. “They make a lot when they infect cells and cause them to explode.”

The exact timing of the tests could also have mattered.

I rushed to the nearest express PCR site just after I started feeling the first twinges of a scratchy throat. It was negative. Not surprising since the first symptoms in vaccinated people are often the activation of your immune system. The next morning I went to my local testing site for a rapid test and PCR. The rapid was negative.

That afternoon, however, the sore throat suddenly got worse. My head started pounding. I did a quick Flowflex test: There was a faint line. I wanted to confirm this with a second test. I took a BinaxNow test done by Abbott Laboratories: negative.

I went to bed thinking the PCR result the next day would confirm the worst. Only it also came back negative. I was puzzled.

The next day I took faster and PCR tests with mixed results. Eventually my diagnosis of Covid-19 was confirmed daily with multiple brands of rapid antigen tests over the next week.

Mara Aspinall, professor of biomedical diagnostics at Arizona State University, says another potential explanation for my results was that there wasn’t enough sample in my swab.

“I think people don’t fully recognize that it’s not mucus you want; it’s the lining of the nose,” says Dr. Aspinall, a board member of OraSure, a maker of rapid tests. Always blow your nose before a test.

False positive rapid tests are very rare. So you can be confident of a positive rapid test no matter how low the line, say doctors and scientists. The weak Flowflex test should have been confirmation enough for me.

“If you’re positive on a PCR or an antigen test, you’re very, very, very likely to be Covid positive,” says Nathaniel Hafer, assistant professor in the University of Massachusetts Chan’s Molecular Medicine Program. Medical School, which is part of a National Institutes of Health project that conducts laboratory testing of rapid home antigen tests.

Dr. Hafer says false negatives are often due to sampling errors.

“There are a lot of things that range from a sample taken from your nose, transported to the lab, and managed by the lab,” says Dr. Hafer. “The process is pretty robust, but sometimes weird things happen.”

So how do you explain the two different results of rapid home tests taken just minutes apart?

Dr Hafer says the threshold of virus needed to make a rapid test positive varies slightly between brands, which could explain why a low amount of virus can result in a weak line in one brand and a negative in another.

Blythe Adamson, CEO and founder of Infectious Economics, a New York-based public health consulting firm that conducts infection prevention activities for private companies, says “with BA.5, we noticed more gaps test than with other variants”.

Part of the problem is that there are more reinfections and some occur as early as three weeks after an infection, she says.

Wilbur Lam, principal investigator of the National Institutes of Health project monitoring the sensitivity of rapid tests to Omicron, says his lab has preliminary data indicating that rapid antigen tests are less sensitive with Omicron compared to other variants. Dr. Lam says that with Omicron, things can change quickly, leading to disparate test results.

“Hours matter,” says Dr. Lam. “All of these Omicron sub-variants are moving fast. Windows are suddenly compressed in time.”

Another abnormality occurs at the end of infections, says Dr. Lam. People with negative PCR tests have antigen tests that come back positive.

Rebound infections – where antigen tests go from positive to negative to positive – appear to occur even in some people who have not taken Paxlovid, the antiviral commonly prescribed to people at risk of severe Covid-19, which may cause kickback.

Dr. Lam said he personally experienced this with a recent infection despite not taking Paxlovid.

A spokeswoman for Abbott said its evaluations, along with studies by independent researchers, have found that the BinaxNow rapid tests can detect both BA.4 and BA.5 variants with comparable sensitivity to previous variants. Indeed, after my first negative test with Binax, it turned out to be positive the following days.

A representative from ACON Laboratories, the maker of Flowflex, could not be reached for comment.

Dr. Adamson says the Flowflex antigen tests seem to detect Omicron variants better than other brands of rapid tests, which may explain why I had a faint line so early with this brand. This may also explain why I continued to test positive on Flowflex for over 10 days.

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