Joseph Prezioso /AFP via Getty Images
How much should you trust the results of a rapid antigen test? That’s a question many people are asking these days amid recent research and anecdotes suggesting these tests may be less sensitive to omicron. Researchers are working fast to figure out what’s going on and how to improve the tests.
That includes people like Dr. Wilbur Lam, a professor of pediatrics and biomedical engineering at Emory University and one of the lead investigators assessing COVID-19 diagnostic tests for the federal government. His research team began evaluating rapid antigen tests against live samples of the omicron variant last December in the lab, and in early assessments, he says, some tests failed to detect the coronavirus “at a concentration that we would have expected them to catch it if it were another variant.”
That finding prompted the Food and Drug Administration to update its online guidance in late December to note that, while rapid antigen tests do detect the omicron variant, “they may have reduced sensitivity.”
A week later, a small preprint study found that in 30 people infected with the omicron variant, rapid antigen tests only detected a positive case two or three days after a PCR test caught it — and “sometimes even longer,” says Anne Wyllie, a microbiologist at Yale School of Public Health and one of the authors of that study.
Rapid tests have always worked best when people are showing symptoms and have high viral loads, and so far, real-world data suggests they’re holding up well on that front. A recent study of 731 people found that the Abbott BinaxNOW rapid tests performed about as well with omicron as they did with other variants when people were symptomatic and had high viral loads.
Lam says that’s also what he’s finding when assessing rapid antigen tests with symptomatic patients who come into the clinic.
“These tests — they work,” Lam says. “When patients come in and they have symptoms …. we test them against the gold-standard PCR test and then we test with [a rapid test]. And by and large, with omicron, we see that they’re performing as expected.”
What’s different now is that with omicron, many people seem to be coming down with symptoms earlier on in an infection — before tests detect a positive case. Anecdotal reports abound of people showing symptoms of COVID-19 and testing negative at first, before eventually testing positive.