Some COVID-19 tests may miss 20% of cases, experts say. Why the false negatives?

Even though some people have all the symptoms of COVID-19, their tests can still come up negative.

Now experts believe there are a lot more false negative tests than experts previously thought, according to NBC News.

The pandemic hit the U.S. hard in March, prompting the Food and Drug Administration to grant emergency use authorization to 70 coronavirus tests, NBC reported. Now, experts are saying those tests could be missing up to 20% of positive COVID-19 cases, according to NBC.

Emergency use authorizations “allow unapproved medical products or unapproved uses of approved medical products to be used in an emergency to diagnose, treat, or prevent serious or life-threatening diseases or conditions caused by [chemical, biological, radiological, or nuclear] threat agents when there are no adequate, approved, and available alternatives,” according to the FDA’s website.

Researchers at Johns Hopkins University also found there is a fairly high rate of false negative tests, Science Daily reported. The chance of a positive case turning up negative is greater than one in five, according to Science Daily.

False positive tests, however, are as rare as 0%, New Hampshire state epidemiologist Dr. Benjamin Chan told the New Hampshire Union Leader.

“We have not seen any significant reports of false positives,” Chan told the Union Leader. “We don’t think that is likely to happen; it’s not common at all.”

Why so many false negatives?

One possibility is how patient samples are collected, NBC reported.

“The false negatives are mainly due to specimen acquisition, not the testing per se,” Dr. Alan Wells, a medical doctor for the University of Pittsburgh Medical Center clinical laboratories and a professor of pathology, told NBC.

Specifically, the method of collection used for polymerase chain reaction (PCR) tests — the most common method — is ripe for error, according to NBC. PCR detects coronavirus genetic material by testing a sample collected with a long swab from the back of a person’s throat, NBC reported.

“You’re sampling blindly. You’re hoping you get the right spot. Then as the disease progresses, the virus might migrate down into your lungs,” Wells told NBC. “You have to be at the right place at the right time.”

Other health experts say that method is the most accurate test for COVID-19, the Union Leader reported. Different testing methods may be even more inaccurate, such as those that collect saliva without using the uncomfortable swab, Wells told NBC.

“The reason for pharyngeal swabs is the virus preferentially infects and replicates starting way back in the inner cavities of the nose and not out in front,” which can result in missing up to 50% of asymptomatic cases, Wells told NBC.

Researchers at Johns Hopkins found “the probability of a false negative result decreases from 100% on Day 1 of being infected to 67% on Day 4,” Science Daily reported. If a person is tested three days after experiencing symptoms — or eight days after being infected — the false negative rate falls to 20%, according to Science Daily.

Even if a person is tested the day they start experiencing symptoms, the probability of a test producing a false negative is 38%, Science Daily reported. But the probability then increases from 21% on Day 9 to 66% 21 days after a person is infected, according to Science Daily.

New Hampshire health experts also say that a patient infected with coronavirus may test negative if they are tested too early, the Union Leader reported. If a person is asymptomatic, they are more likely to get a false negative, according to the Union Leader.