A fresh study by UCSF researchers based on data from a walk-up test site in San Francisco found that Abbott's popular BinaxNOW rapid antigen test picked up nearly all of the most highly infectious cases of the Omicron variant, indicating it is a highly useful test for preventing spread of the virus.
Questions have been raised for weeks about whether at-home antigen testing was fully picking up infections with the highly contagious Omicron variant. And the answer, according to a just-released paper from UCSF researchers and the Chan Zuckerberg Biohub, is no — however the the BinaxNOW test is very reliable in picking up the most infectious cases with the highest viral loads.
While the rapid test may have missed a third of cases that were later picked up by PCR tests, the antigen test did pick up 95% of cases with the highest viral loads — and 98% of all symptomatic cases.
"It’s working as it was designed,” said Joseph DeRisi, a UCSF biochemist and co-president of the Chan Zuckerberg Biohub, speaking to the New York Times. “There does not seem to be any performance deficit with Omicron."
The caveat is that the BinaxNOW test only picked up 65% of the total number of cases in the study that were picked up in PCR testing, and those cases it missed appear to have largely been asymptomatic and likely less infectious cases.
The study is based on 731 people who came for testing at a walk-up site in San Francisco on January 3 and 4. Each person was given two nasal swabs — one of which went into a BinaxNOW kit, and one of which was sent for PCR lab testing. And the researchers hope this will quell some of the mixed messaging that has been coming from health officials about what type of tests people should get if they think they were exposed.
Because of the deficit in asymptomatic or low-viral-load cases being detected, repeat testing is recommended for high-risk individuals. But otherwise, the data in this small-scale study confirms what was seen in other real-world studies of antigen testing prior to the emergence of the Omicron variant, the researchers say.
As the Times notes, the acting commissioner of the Food and Drug Administration, Dr. Janet Woodcock, was just commenting about the accuracy of at-home tests during testimony before a Senate committee today.
"We believe all of [the tests] detect Omicron," Dr. Woodcock said. "We simply feel they are somewhat less sensitive than they were to some of the previous variants."
Dr. DeRisi explained to the Times that of those tests that came back positive via PCR testing with lower viral loads, but negative on the BinaxNOW test, those patients were likely either very early in their infection, or they were already recovering, and therefore there was less virus in their noses.
Recent advice has suggested that swabbing the throat in addition to swabbing nostrils might be necessary to pick up the Omicron variant, but this study did not compare that method.
An even smaller-scale, also not-peer-reviewed study emerged last week suggesting that the BinaxNOW test and another at-home test were liable to fail to detect positive cases even with high viral loads. But that study focused only on 30 patients in one workplace, and appears to be contradicted by UCSF's paper. However that other study suggests that there could be a spread of several days between when a person's viral load becomes detectable on the antigen test, and that an immediate PCR result is therefore helpful in preventing a person from unknowingly transmitting the virus. The trouble with this is clear: getting a PCR result back often takes several days in the real world, especially at a moment of backlog at labs.