Misuse of home COVID tests could lead to viral spread

BOSTON — They were the bad old days of a COVID-19 diagnosis. Long lines at designated testing sites. A shortage of home tests. But, for the moment, that seems behind us.

Increasingly, Americans are conducting their own clinical assessments when it comes to the virus – for better and worse.

“Ninety percent of tests are over the counter, and maybe 10 percent are lab-based PCR tests,” said Nate Hafer, PhD, an assistant professor in the Program In Molecular Medicine at the UMass Chan Medical School in Worcester.

While Hafer still sees a place for PCR tests, he notes the downsides:

“They’re much more expensive,” he said. “And the time involved in getting the results is one or multiple days.”

By contrast, home tests are cheap, easy to use, convenient and they give quick results.

Just one problem with those quick results: they can be wrong, if the tests are used too early in an infection.

“The way this virus replicates is it usually takes off in the first 24 to 48 hours – the viral load in the nose,” said Sanjat Kanjilal, MD, associate medical director of Microbiology at Brigham and Women’s Hospital.

The problem with at-home antigen tests is that a rather significant viral load is needed to detect infection. That’s unlike PCR tests, which first amplify what virus is present. This allows PCR tests to pick up even tiny amounts of genetic material.

“Antigen tests are always going to be one to two to three days later in picking up COVID infection than a PCR test,” Hafer said.

That’s why it’s critically important consumers use both tests contained in a kit, he said, about a day apart. Used correctly in this way, Hafer said the accuracy of home tests rivals that of the PCR.

But clearly, some are ignoring these “best practices” when it comes to home testing – even though they’ve been publicized for months.

“If you catch someone early in that growth curve, you still may be below the level of detection for the antigen assay – and that’s not new,” Kanjilal saiid.

“What is a question: have those replication kinetics – the replication curve – changed with Omicron or changed in the presence of people who have vaccination? I think that answer is not really known.”

What is known is that home COVID tests have little trouble detecting Omicron – once enough virus is present, said Hafer, who co-authored a study on the subject. And the detection delay on Omicron – one to three days – is similar to that of the Delta variant.

This would appear to hold true for the Omicron sub-variants that have circulated since last winter.

There’s also optimism the tests will hold up against even newer variants that might arise heading into fall – if the virus continues its somewhat predictable pattern of mutation.

“Many of the over-the-counter antigen tests are against the nucleocapsid gene in the virus,” Hafter said. “And that has tended to mutate less than other parts of the virus, such as the spike protein.”

The most serious potential problem with home COVID testing is when a symptomatic person concludes they do not have Covid, based on a singular home test.

“That is the point where you have to say no,” said Nitika Pant Pai, MD, MPH, PhD, an associate professor in the Department of Medicine at McGill University, Montreal. “I’m symptomatic. I’m getting a negative test. I need to confirm this.”

Pai points out that antigen tests done at home are really more a screening tool for the virus, whereas the PCR is a confirmatory test.

“If you think you have infection because you’re symptomatic or exposed, then you really should get the most sensitive test possible to answer the question as quickly as possible,” said Kanjilal. “Which would be a PCR.”

In the past, that might have meant spending hours in line.

But times have changed dramatically when it comes to COVID testing – at least for now.

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