HOW COMMON IS A FALSE-POSITIVE COVID TEST?

Fact checked by Nick Blackmer

Key Takeaways

A new report showed that people who do not have COVID may still get recurring false-positive rapid test results.
Although false-positive test results can occur with COVID rapid tests and PCR tests, experts say they are very rare.
Not doing the test correctly, contamination, and cross-reactivity are all factors that can make it more likely you’ll get a false-positive result.

Even if you’re not actually infected with COVID, there’s a tiny chance you could still get a false-positive rapid test result.

A brief report published last week in the New England Journal of Medicine found that in a group of 11,297 people who tested themselves for COVID every 48 hours for 15 days, about 1.7% received at least one false-positive result on a rapid test—even though they had negative molecular tests done at the same time.

Researchers identified a false positive when a participant got a positive result on a rapid antigen test but had a negative RT-PCR test that was taken within 48 hours. Of the 191 participants who got false-positive results, only 13 of them had persistent false-positive results.

Here’s why might you get a false-positive COVID test and which factors could make you one of the rare few who gets them persistently.

False-Positive Rapid Tests Are Rare

It’s extremely uncommon to repeatedly get a false-positive result on a COVID rapid antigen test, Charles Bailey, MD, medical director for infection prevention at Providence St. Joseph Hospital and Providence Mission Hospital, told Verywell.

While 1.7% of patients in the study had at least one false positive antigen result, only 0.1%—or 1 in 1,000—had persistent false positive antigen test results.

“It is very rare, as noted,” said Bailey. “Therefore, these are not likely to be of broad importance or concern.”

In general, false-positive COVID test results do not happen often because of the high specificity of the testing methods used, Heba Mostafa, PhD, MBBCh, associate professor of pathology at the Johns Hopkins University School of Medicine and director of the Molecular Virology Laboratory at the Johns Hopkins Hospital, told Verywell. Specificity is a test’s ability to correctly rule out when someone does not have a particular disease or condition. The higher the specificity of a test, the lower the likelihood of a false-positive result.

“They are quite uncommon,” said Mostaga. “Diagnostic assays, whether rapid or molecular have proven very high specificity.”

Who Is More Likely to Get a False-Positive Test Result?

We don’t really have enough data to say for sure who is the most likely to get a false-positive COVID test result. However, in the recent report, most of the participants who had persistent false positives were women, had some kind of autoimmune disorder, and used a specific brand of test.

People With Autoimmune Conditions

“This article, which utilized a large cohort of data, highlighted that certain autoimmune conditions were associated with false positivity, specifically with one brand of rapid tests,” said Mostafa. “Rheumatoid factor has been reportedly linked to false positive immunoassays.”

Rheumatoid factor is a protein in the blood of people with rheumatoid arthritis (RA), said Bailey. It’s known for reacting with different tests like immunoglobulin (IgM) tests, which measure the amount of antibodies in the blood.

“Those people with circulating rheumatoid factors might be at risk for false-positive results from a variety of tests, including COVID assays and some assays for a variety of other infections such as Valley fever or syphilis,” said Bailey.

Women

That could also explain another finding from the report—that most of the persistent false positives were in women. Dean Winslow, MD, professor of medicine at Stanford University, told Verywell that this might explained by the fact that “women have a higher prevalence of…rheumatoid factor than men do.”

People Who Used a QuickVue Test

A final factor is that the occurrence of false-positive results in antigen tests in the study was also associated with a particular test brand—the QuickVue rapid antigen test. The reason why a particular test might produce a false-positive result is because of how it’s designed and reacts to certain substances, according to Bailey.

“Persistent false-positive antigen test results seem to be associated with one particular brand of antigen test and therefore might relate to the specific detection threshold (the amount of “target” antigen protein that yields a positive result) or the test’s sensitivity to a cross-reacting substance (usually another protein such as rheumatoid factor),” said Bailey.

The authors of the report wrote that patients with persistent false-positive results and a history of autoimmune disease with rheumatoid factor may benefit from repeat COVID testing with a different brand of rapid antigen test.

Why Do False-Positive Test Results Happen?

While some people may be predisposed to getting a false positive again and again, Mostafa said most COVID false positives are likely one-offs and the result of one of the following factors:

Doing the test incorrectly: User errors like not swabbing the nasal cavity enough, not mixing the solution correctly, or not waiting the recommended time to check the test can all lead to inaccurate results.
Contamination: If the sample or testing equipment is contaminated during collection or handling, it can lead to inaccurate results.
Cross-reactivity: Some tests may react with other substances in the body, like a similar virus or a non-specific protein, resulting in a false positive result.
Testing too early: In some cases, testing too early in the infection when the viral load is low can also affect the test’s accuracy.

If you get a positive result on a rapid antigen test but are unsure if the result is accurate, Winslow recommends testing again. You can also contact your healthcare provider to get a PCR test—considered the “gold standard” for COVID tests—or other molecular tests.

“I think it would be important for individuals to get a PCR test or some other molecular test just to see if the antigen test is a false positive,” said Winslow. “Again, it would depend on the specific circumstance whether you’d want to follow it up.”

What This Means For You

If you get a persistent false-positive test result and have a history of autoimmune disease with rheumatoid factor, experts say that you should consider repeat testing. You may also want to ask your healthcare provider about getting a PCR test or other molecular test to confirm the accuracy of your rapid test result.

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.

Read the original article on Verywell Health.

2024-03-01T18:30:46Z dg43tfdfdgfd