When to trust and question your COVID test results

After dodging COVID-19 for almost 2.5 years, I knew my time would come. But after all those conflicting test results, I was stunned.

How to interpret the winning results? I asked some doctors, academics and scientists what they said:

First, test discrepancies with Omicron and its subtypes seem to be increasingly common, so some common sense comes in handy. If you’re like me and have been living with people with COVID-19 and feel like they develop symptoms, you’re probably developing COVID-19. So even if your tests say something else, but stay at home.

For rapid antigen tests, serial testing is the name of the game. Test every day or every other day for a week if you can. Once you are positive you can count on it, even if it is a faint line.

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

My negative PCR test falls in that small group. I noticed that the test site I was using was not thorough. They were definitely not counting the 15 seconds per nostril that I did at home and barely pushed my nose up.

If they didn’t look carefully they would have missed a very faint line.

“There will be some amount of error in everything you need to do,” says Gigi Gronwall, a senior scholar 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 spread rapidly to my nose, resulting in my faint positive line eight hours later.

“Viruses multiply rapidly. They make a lot of themselves when they infect cells and blow them up,” says Dr. Gronwall.

The exact timeline on the tests could also matter.

I rushed to the nearest Express PCR site soon after I felt the first tremors of a sore throat. It was negative. This is not surprising because the first symptoms in people who are vaccinated are often the activation of your immune system. The next morning, I went to my local testing site for a rapid and PCR test. The uptrend was negative.

That afternoon, however, the scratchy throat suddenly felt worse. My head started twitching. I took the Flowflex Rapid Test: There was a faint line. I wanted to confirm this with another test. I took a BinaxNow test done by Abbott Laboratories: negative.

I went to bed and thought the next day the PCR result would be the worst. Only this also came negative. I was surprised.

The next day, I did more rapid and PCR tests with mixed results. Eventually, my COVID-19 diagnosis was confirmed over the next week with multiple brands of rapid antigen tests on a daily basis.

Another possible explanation for my results was that my swab didn’t have enough sample, says Mara Espinol, a professor of biomedical diagnostics at Arizona State University.

“I think people don’t fully recognize that this isn’t the mucus you want; it’s the lining of the nose,” says Dr. Aspinall, who is on the board of OraSure, the maker of the rapid test. Always blow your nose before the test.

False positive rapid tests are very rare. That’s why you can count on a positive rapid test, say doctors and scientists, no matter how blurry the line is. The faint flowflex test should have been confirmation enough for me.

“If you’re positive on a PCR or antigen test, you’re very likely to be positive for COVID,” says Nathaniel Hafer, an assistant professor in the program in molecular medicine at the University of Massachusetts Chan Medical School. Part of a National Institutes of Health project that conducts laboratory tests of rapid antigen tests at home.

Dr. Hafer says that false negatives are often caused by sampling errors.

“The sample being collected in your nose is taken to the lab, and run in the lab,” says Dr. Hafer. The process is very robust but sometimes strange things happen.

So how to interpret two different at-home rapid test results in minutes?

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

Blythe Adamson, chief executive and founder of Infectious Economics, a New York City-based public-health consulting firm that specializes in infection prevention for private businesses, says, “With BA.5 we found more test anomalies than with the other variants. Have seen.”

Part of the issue is that more re-infections are happening and some are happening three weeks after infection, she says.

Wilbur Lamm, who is the lead investigator of the National Institutes of Health project monitoring the sensitivity of the rapid test to Omicron, says his lab has preliminary data indicating that rapid antigen testing with Omicron is comparable to that of other variants. are less sensitive. Dr. Lam says things can change rapidly with Omron, resulting in uneven test results.

“Hours matter. All these omicron subvariants are moving rapidly. The windows suddenly get narrower in time,” says Dr. Lamm.

Another discrepancy is occurring at the later end of the infection, says Dr. Lamm. Antigen tests become positive again in people with negative PCR tests.

Rebound infections – where antigen tests go from positive to negative back to positive – also occur in some people who did not take paxclovid, an antiviral usually prescribed for people at risk of severe COVID-19, which can result in rebound .

Dr. Lam said he personally experienced that with a recent infection despite the fact that he didn’t take paxlovid.

An Abbott spokesperson said its assessment, as well as studies by independent researchers, found that the BinaxNow rapid test can detect BA.4 and BA.5 variants, which can be compared as the former variants. In fact, after my first negative test with Binex it turned out to be positive the following days.

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

Dr. Adamson says the Flowflex antigen tests seem to be better at detecting Omicron variants than rapid tests from other brands, which may explain why I found a faint line with that brand so quickly. This may also explain why I continued to test positive on Floflex for more than 10 days.

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