Coronavirus Tests May Display False Negative Results If Taken Too Early, Hopkins Study Warns


Researchers say the best time to take a COVID-19 test is about eight days after being infected (or three days after symptoms appear).


BALTIMORE — Testing has been proclaimed time and time again over the past few months as an integral part of getting COVID-19 under control. After all, the only way we’ll know the extent of infections is if tests are readily and widely available. Unfortunately, a new study is calling into question the validity of many coronavirus test results.

Researchers from Johns Hopkins Medicine say that individuals tested too early after contracting COVID-19 are likely to return a false negative test result. These findings apply to tests relying on viral genetic material only.

“A negative test, whether or not a person has symptoms, doesn’t guarantee that they aren’t infected by the virus,” says Dr. Lauren Kucirka, an obstetrics and gynecology resident at Johns Hopkins Medicine, in a release. “How we respond to, and interpret, a negative test is very important because we place others at risk when we assume the test is perfect. However, those infected with the virus are still able to potentially spread the virus.”

Researchers say that people who probably came into contact with the coronavirus should be treated as if they are infected. That’s the case regardless of what an early test result says, and especially if they display any relevant symptoms.

COVID-19 Tests Relying On Viral Genetic Material

Reverse transcriptase polymerase chain reaction (RT-PCR) tests are the most frequently used coronavirus tests right now, mostly because they can produce a result very quickly. These tests function by copying the virus’ genetic material and then using that copy to detect an active infection. However, while the tests do work quickly, they are also prone to displaying false negative results. If a swab misses infected cells, or if viral levels are very low early on in an infection, the tests have been known to display an inaccurate reading.

Despite this, RT-PCR tests are being used in nursing homes and hospitals all over the country.

For their research, the team at Johns Hopkins analyzed RT-PCR test data from seven earlier projects. All of that data encompassed 1,330 respiratory swab samples from both hospital patients and others identified using contact tracing. Those results were then cross-referenced against when patients reported encountering the virus and/or when symptoms first appeared.

Using this approach, the study’s authors estimated the probability that someone infected with the coronavirus would display a false negative test result.

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They conclude that individuals tested within four days after being exposed to the virus were 67% more likely to test negative despite being infected. For people already exhibiting coronavirus symptoms, that percentage drops to 38%.

Don’t Rush To Get Tested, Doctors Suggest

The best time to be tested, according to the study, is about eight days after being infected (or three days after symptoms appear). However, even in these cases patients still have a false negative rate of 20%.

“We are using these tests to rule out COVID-19, and basing decisions about what steps we take to prevent onward transmission, such as selection of personal protective equipment for health care workers,” Kucirka says. “As we develop strategies to reopen services, businesses and other venues that rely on testing and contact tracing, it is important to understand the limitations of these tests.”

The study is published in Annals of Internal Medicine.

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