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It may be best to repeat an early COVID-19 antigen a test a day or two later for symptomatic individuals who have negative results, the authors of a new study say.

Investigators from federal and state health agencies sought to examine how home antigen test efficacy might change over the course of a SARS-CoV-2 infection. More than 200 study participants took antigen tests for 15 days and also received at least one swab for a RT-PCR test, viral culture, and sequencing. 

The sensitivity of these tests peaked at about four days after symptom onset, the researchers reported. On day six of illness, more than half of the participants had positive antigen test results. By day 11, that proportion had dropped to one-fifth of participants.

“Sensitivity improved when a second antigen test was performed one to two days later, particularly early in the illness course,” they wrote. “These findings suggest that symptomatic individuals with an initial negative home antigen test result should test again one to two days after,” they concluded.

Overall antigen test performance was judged “moderate” when compared to confirmed RT-PCR test results.

The study took place before the omicron variant emerged. But another study from March 2022 comparing the effectiveness of BinaxNow antigen tests with that of RT-PCR tests found that antigen tests perform well with omicron infections, they noted. 

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