ROCHESTER — With long lines at COVID-19 screening centers, the demand for at-home testing is high. But the real-world effectiveness of the rapid antigen kits is under question, and the impact of a positive diagnosis is changing as well.
Rapid antigen COVID-19 tests are popular because, in just minutes, they can identify antigens, or small viral proteins signaling infection.
Nationally, the use of such kits is believed to dwarf that of tests ordered through a lab.
When they can be found in stock at drug stores and online, the tests are scooped up at high prices, limiting access. Late last month the Biden administration announced a goal of making 500 million such tests free to all Americans, though their arrival could take months.
The home kits have relieved pressure at testing centers. But because the results are reported to private individuals and not the state, some experts worry the tests have diminished the ability for health officials to track the prevalence of COVID-19.
Minnesota health officials have supported their use regardless.
"There are many public health benefits to having widespread rapid testing, as long as people do what they are supposed to do with their results," Doug Schultz of the Minnesota Department of Health said in an email. "But the downside is that you do lose something as far as counting cases."
Schultz says that health officials have always faced unknowns in tracking illness, and have other ways to estimate the spread of omicron, including through data reflecting hospital admissions and clinic visits.
Less encouraging about the at-home testing craze is the way that omicron appears to have diminished the reliability of negative results on antigen tests, as well as the effectiveness of treatments available to those who worsen after infection.
Antigen rapid tests less accurate early
A long-known drawback to rapid antigen tests is that they sometimes will miss a true case of COVID-19, wrongly reporting that you are not infectious. This is called a false negative, and new research suggests it likely happens to a degree that can promote spread.
Before omicron, the best kits offered a 15% rate of false negatives, with other studies showing a 40% rate for missing asymptomatic COVID-19. According to the FDA, omicron has likely made these percentages worse. To what degree was unknown.
That picture became clearer on Wednesday, Jan. 5, when the first real-world study of rapid take-home testing during the age of omicron suggested a three-day lag between when people became highly infectious, and the point at which rapid antigen tests spot the variant.
Reporting in the pre-print journal medrxiv.org — a platform for research that has not yet been peer-reviewed and therefore should be read with caution — researchers described a 30-person sample of vaccinated workers given daily sets of saliva PCR and nasal rapid antigen tests during December of 2021.
The authors believe that the three day lag to spot COVID-19 in rapid antigen tests was noteworthy for the high level of infectiousness early in the illness.
"The part that was alarming was the viral load of the people in those days before they tested antigen positive," said lead author Blythe Adamson, infectious disease epidemiologist and economist "We confirmed there were real transmissions that happened ... when people were antigen negative."
Adamson says rapid antigen tests remain valuable tools as long as their risk of early false negatives with omicron is kept in mind.
"We can't rely too heavily on a negative rapid antigen test meaning that we are not infectious to other people," she said. "Rapid antigen tests are still an incredibly valuable tool for people who've had a couple days of symptoms, who maybe they feel like they've had allergies for a few days."
Two lines — now what?
The good news is, if they sometimes miss omicron cases early in the course of illness, rapid antigen tests are unlikely to deliver a positive finding in error, with specificity accuracy ratings in the 99% range.
"If you get a positive (antigen) test it usually, not always but usually, is a true positive," said Mayo Clinic vaccinologist Dr. Greg Poland.
Much like a pregnancy test, a positive result from a nasal swab produces two lines on a test strip.
The latest CDC guidance says if you are fully vaccinated, a positive result to a COVID-19 test should lead to masking and isolation within the home for a period of five days. This period may warrant a call to your health care provider, if not necessarily a visit.
"Just because you have a positive test does not necessarily mean you have to interact with the medical system," Poland said. "There isn't enough medical system to handle everyone who would have a positive test."
If you are low risk, a positive diagnosis during omicron is a lonely bout of masking, TV and over the counter symptom management..
"Somebody who is young, somebody who is healthy without co-morbidities," Poland said, "somebody who has been vaccinated, anybody who has mild symptoms that are not progressing, they really don't need any interaction with the medical system other than to notify their health care provider if they (get) new symptoms that might be persisting."
The early days following a positive result are critical, on the other hand, for those at risk of poor outcomes from an infection.
For the elderly, immune-compromised, and the unvaccinated and metabolically unhealthy, early use of monoclonal antibodies made by Lilly and Regeneron had been effective at reducing hospitalization.
Those no longer work as well with the omicron variant, however. According to Poland, newer monoclonal antibodies targeting omicron are in short supply due to the surge of new cases.
Paxlovid, an oral antiviral treatment for COVID-19, is expected to reduce omicron hospitalizations, but when faced with sick patients needing early treatments, variant type is not always known to doctors.
In this way, while omicron may turn out to be milder overall, the tools for treatment are fewer. In the meantime, the triggers for seeking help after testing positive at home remain the same.
For the CDC, the symptoms that warrant emergency attention include trouble breathing, persistent pain or pressure in the chest, new confusion, inability to wake or stay awake, and pale, gray, or blue-colored skin, lips, or nail beds, although skin tone can affect visibility of those symptoms.