A negative rapid test does not mean you are free of COVID


Julie Ann Justo, an infectious disease clinical pharmacist for a South Carolina hospital system, hoped that Christmas week would finally be the time her family could safely reunite.

Before the celebration, eligible family members were vaccinated and had their boosters. They isolated themselves and wore masks. And many relied on the negative results of the rapid covid tests that were done before the meeting that brought together 35 people in South Florida.

But in less than a week, Justo and at least 13 members of his family tested positive for Covid, with many experiencing the typical symptoms of an upper-respiratory virus, such as a sore throat and a runny nose.

Like many others, Justo’s family learned the hard way that a single negative result from a home test is no guarantee that a person is not sick or carrying the virus.

That is even more true with the omicron variant, which is highly contagious. Testing can be done too soon, before enough virus is present to detect it, or too late, after a person has already infected others.

And most tests are supposed to be used in pairs, to take two tests a few days apart from each other. But many brands bring only one kit, and the cost plus the scarcity mean that many take only one test.

While experts say home antigen tests remain a useful tool, they add that they can provide false confidence.

Some people mistakenly view home tests “as a freedom card,” said Dr. William Schaffner, an infectious disease specialist at Vanderbilt University School of Medicine in Nashville, Tennessee. “I’m negative, so I don’t have to worry anymore.”

“Omicron is so transmissible that it’s a challenge to use any type of testing strategy in terms of going or not going to meetings and being successful,” said Dr. Patrick Mathias, vice president of clinical operations in the department of laboratory medicine and pathology at the School of Medicine. of Medicine from the University of Washington.

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Rapid tests are pretty good at correctly detecting infection in people with symptoms, Mathias added, with an accuracy range of 70% to nearly 90%, an estimate from several studies.

Other research, some before the current variants circulated or in more controlled settings, has shown higher rates, but tests may still miss some infected people.

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This increases the risk of spread, and the chance grows dramatically depending on the number of people attending an event.

Rapid tests are less accurate in people without symptoms.

For the asymptomatic, rapid tests “on average correctly detect infection about 50% of the time,” said Shama Cash-Goldwasser, an adviser to Prevent Epidemics at Resolve to Save Lives, a nonprofit group run by the former CDC Director Tom Frieden.

Looking back, Justo said his family did everything they could, but acknowledges that two things put them at greater risk: Not all family members were tested before getting together due to a shortage of kits. And there were several children under the age of 5 who are not yet eligible for the vaccine. Later, they were among the first to show symptoms.

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“We are probably too reliant on negative rapid tests to gather indoors, without other layers of protection,” he said.

“One critical thing is the timing of the test,” Schaffner said. If the test is done too soon, such as a day or two after exposure, the results may not be accurate. Likewise, testing several days before an event won’t tell much about who may be infectious on the day of the party.

Schaffner and others recommend starting testing three days after a known exposure or, if you feel sick, a few days after symptoms start. It is always a good idea to use both tests in the kit, according to the instructions, the second 24 to 36 hours after the first. For an event, make sure that one of the tests is done on the day of the event.

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Antigen tests work by looking for specific proteins in the virus, which must be present in adequate amounts for the test to detect them. (Lab PCR tests are more accurate because they can detect smaller amounts of the virus, but they take longer to return results.)

Covid markers can remain as remnants long after the live virus has disappeared, which is why some scientists question the use of tests, either antigen or PCR, as a measure of when patients can end their isolation, particularly if they seek to shorten the recommended period.

The CDC recommends five days of isolation, which can end if symptoms clear up or resolve, without a fever.

Some patients will test positive 10 days or more after the first symptoms, although it is unlikely that they will still be infectious by then.

Still, that means many people are using rapid tests inappropriately, not only relying too much on them as protection against Covid, but also as an indicator of when an infection is over.

Rapid home tests should be used for several days to increase the chances of getting an accurate result.

“Each individual test doesn’t have as much value as serial tests,” said Dr. Zishan Siddiqui, medical director of the Baltimore Convention Center Field Hospital and an assistant professor of medicine at Johns Hopkins University. And, because the test is less accurate in people without symptoms, he said asymptomatic people shouldn’t rely on a single rapid test to reunite with friends or family without other mitigating measures.

One study looked at 30 adults vaccinated in December 2021. “Most omicron cases were infectious for several days before becoming detectable by rapid antigen tests,” the study found. The research has not yet been peer-reviewed.

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False negatives are also more likely when there is community spread and virus circulation is rampant, as is now the case in most states.

“If there’s a lot of community spread, that increases the likelihood that you’ll have COVID,” Cash-Goldwasser said. However, the tests, statistically, will still miss a certain percentage of those actually infected. With a higher number of people likely to be infected, and the same percentage chance of missing cases, the absolute number of false negatives increases, he explained.

So at this point, “if you have a negative result, it’s important to be more suspicious,” he said.

Vaccinations, boosters, wearing masks, social distancing, ventilation, and separate testing are all imperfect solutions to prevent infection. But layered, they can serve as a more effective barrier, Schaffner said.

“Rapid testing is helpful” — his own family used them before getting together for Thanksgiving and Christmas — “but it’s a barrier with holes in it,” he added.

The Justo family tried those layers, but there were gaps through which the virus passed. While most family members had mild symptoms, Justo said she felt short of breath, felt fatigued, and had headaches, muscle aches, and nausea. About 10 days passed before he felt better.

“I certainly spent a lot of time reviewing what we could have done differently, but the reality was that we did the best we could,” Justo said. “Fortunately no one needed to go to the hospital, and I attribute that to the vaccinations. For that I am grateful.”

KHN (Kaiser Health News) is the newsroom of KFF (Kaiser Family Foundation), which produces in-depth journalism on health issues. Along with Policy Analysis and Surveys, KHN is one of the three main programs of KFF. KFF is a nonprofit organization that provides health information to the nation.

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