Recent research suggests that rapid tests may be less sensitive towards omicron than previous coronavirus variants, increasing the risk of false negative test results.
Covid-19 testing and accuracy
Although real-world data indicates rapid Covid-19 tests continue to reliably detect the omicron variant among symptomatic patients with high viral loads, some early data suggests these tests may be less reliable at detecting omicron during the early days of infection.
For example, a small, preprint study of 30 people found that rapid Covid-19 tests often remained negative for several days after an infection was detected by PCR tests. In addition, the researchers documented four cases in which an infected person actually transmitted the coronavirus while their rapid test were still reporting negative results.
Similarly, FDA in December cautioned that while rapid Covid-19 tests "do detect the omicron variant, [they] may have reduced sensitivity" and may be less likely to pick up early infections from omicron compared with molecular PCR tests.
According to Wilbur Lam, a professor of pediatrics and biomedical engineering at Emory University and a lead investigator assessing Covid-19 diagnostic tests for the federal government, there are several possible reasons why rapid tests appear to be less accurate with omicron.
One possibility is that vaccinated people's immune systems start fighting off infection the moment it occurs, which means symptoms will occur before viral loads are high enough to be detected by a test. "[E]ven though the virus may actually be living in the patient's nose, the immune system might already be fighting it off, such that the viral load at that point in time of testing is too low to be dateable on the test," said Lam.
Other possibilities include omicron replicating in different areas of the body first (throat vs. nose) or that some omicron subvariants produce fewer antigens, which would make them harder to detect on current rapid tests.
However, according to MedPage Today, it's not only rapid tests that may have accuracy issues. Even PCR tests, which are considered the "gold standard," may return inaccurate results, particularly false negatives.
Daniel Rhoads, vice chair of the microbiology committee at the College of American Pathologists, said PCR sensitivity for detecting the coronavirus is around 80%, meaning "one in five people would be expected to test negative even if they have Covid."
PCR sensitivity is also affected by the prevalence of a disease in an area, MedPage Today reports. The more prevalent a disease is, the fewer false positives there will be, but false negatives will also increase.
"If everybody you know has Covid, you have symptoms of Covid, you test negative [on a PCR test], the prevalence in your community is high, your clinical symptoms align with the disease ... you're at a high likelihood of having a falsely negative result," Rhoads said.
How to use rapid tests for the most accuracy
The accuracy of rapid tests is likely to improve as researchers learn more about potential variations in the coronavirus proteins targeted by the tests, said David Walt, an expert in medical diagnostics at Harvard University. Learning about these variations would allow rapid test manufacturers to target proteins that don't change, which could make the tests "sensitive across the board for all the kinds of variants that they encounter," Walt said.
In the meantime, experts have suggested several ways to ensure you get the most accurate results from your antigen tests.
The best time to use a rapid test is when you're experiencing symptoms, since that's when they are most accurate. According to a recent study of 731 people, Abbott's BinaxNOW rapid tests performed comparably with omicron to results with previous variants when individuals were symptomatic and had high viral loads.
But if you are symptomatic and have a limited number of tests, Michael Mina, an epidemiologist and the chief science officer at eMed, recommends "assum[ing] that you are omicron-positive," but "[w]ait a day, maybe two into symptoms to use your test because people are becoming symptomatic a day or so before they're turning positive."
Experts also recommend that people test themselves frequently. If you're symptomatic but test negative on an initial test, wait a day or two before you test yourself again. According to Walt, after two days, "the virus will have replicated and your viral load will then be high enough to give you a positive result."
If you're symptomatic and still negative after two rapid tests, Lam suggests either testing a third time with a rapid test or getting a PCR test if one is available.
Finally, if a rapid test returns a positive result, it is likely a true positive, MedPage Today writes, and people should isolate themselves. According to Rhoads, rapid tests have a specificity of over 99%, meaning false positives are quite rare.
Overall, if you're not symptomatic, a negative result on a rapid test can be reassuring, but "it's not just an instantaneous free pass," said Bruce Tromberg, who leads the NIH's RADx program aimed at increasing the United States' testing capabilities. Even with a negative test, people should continue to take precautions, such as masking and reducing social contact, when possible, especially if they're going to be around more vulnerable individuals. (Fiore, MedPage Today, 1/21; Godoy, "Shots," NPR, 1/23)