Rapid, but reliable? Early data suggests home tests lag with omicron

Amid surging demand for rapid COVID-19 antigen tests, a small real-world study suggests they may not be a reliable tool to curb transmission. 

Preliminary results published Jan. 5 in MedRxiv indicated people infected with the omicron variant were contagious for several days before rapid antigen tests detected a positive result. Researchers conducted a retrospective study among 30 people diagnosed with COVID-19 between Dec. 1 and Dec. 31 in what were likely omicron outbreaks across five workplaces in New York, Los Angeles and San Francisco. The participants underwent both polymerase chain reaction testing — considered the gold standard of COVID-19 diagnostics — and rapid antigen testing. 

Findings showed it took an average of three days for the rapid antigen test to detect infection after a participant's first positive PCR test. Four of the participants transmitted the virus to others between the time they received a false-negative rapid result and positive PCR result, researchers said. 

"Rapid antigen tests lagged in the ability to detect COVID-19 during an early period of disease when most individuals were infectious with omicron and four transmissions were confirmed," researchers, affiliated with the University of Washington in Seattle and New Haven, Conn.-based Yale School of Public Health, said. "The policy implication is that rapid antigen tests may not be as fit-for-purpose in routine workplace screening to prevent asymptomatic spread of omicron, compared to prior variants, given the shorter time from exposure to infectiousness and lower infectious doses for transmission." 

The preprint study findings arrive while many Americans struggle to even get their hands on rapid antigen test kits as the omicron variant drives people to seek diagnostic tools. The White House is finalizing contracts for 500 million rapid COVID-19 tests that Americans can request through a government website and obtain for free. White House spokesperson Jen Psaki said the first delivery of the tests will occur in late January. That may be too late for consumers' peak demand, given recent projections showing the omicron wave "to be sharp and fast," with models forecasting cases and hospitalizations peaking by the end of January.

Retailers are also upping prices for home kits. Just this week, Kroger and Walmart raised prices for one of the most popular rapid antigen tests, Abbott's BinaxNow, after the expiration of a deal with the White House to sell the test kits at cost for $14. Kroger and Walmart will now sell the kits for $23.99 and $19.98, respectively. 

The findings also coincide with recent debate around the CDC's updated quarantine and isolation guidelines. One of the biggest sticking points has been the CDC's recommendation that Americans infected with COVID-19 end their five-day isolation without a negative test. Rochelle Walensky, MD, CDC director, said questions around rapid antigen testing contributed to the omission of a negative result in the agency's new guidance. The agency declined to add testing as a last step this week.

"Many are asking why we do not require a test at the end of the five days of isolation for those who are infected," she said Dec. 29. "We know that PCR testing would not be helpful in this setting, as people can remain PCR positive for up to 12 weeks after infection and long after they are transmissible and infectious. We also don't know that antigen tests give a good indication of transmissibility at this stage of infection." 

Overall, doubts raised by the small real-world study on rapid antigen tests join a number of ways the omicron variant is challenging the familiar resources and methods to manage COVID-19 and revealing their weak spots. For instance, public health experts have called for greater focus on hospitalizations versus cases during this wave to gauge virus severity. That data point is less reliable as hospitals see more patients with incidental COVID-19 cases, or patients who were primarily admitted for other ailments and test positive during routine screening. 


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