Covid, Study: After a negative home swab, it’s best to repeat it
Home antigen swabs are reliable enough to detect SarsCoV2 positivity, yet their maximum efficacy is concentrated between the third and fourth day after symptoms appear. Therefore, if, after a negative smear, symptoms are present, it may be advisable to have a second test while waiting a day or two, to definitively rule out infection. These are the data that emerge from a study coordinated by the US Centers for Disease Control and Prevention and published in JAMA Internal Medicine. The research included 225 American adults and children who had been confirmed positive for SARS-CoV-2 infection by molecular swab and who also underwent repeated tests at home spontaneously. On average, a home swab was significantly less effective than a molecular swab, but the difference between the two tests diminished if the home-made swab was used in the correct time window: between 3 and 4 days, in fact, the swab capacity was as much as the antigen buffer to detect positivity Correctly 77%. However, the efficacy was found to decline rapidly: in the researchers’ tests, six days after the onset of symptoms, 86% of the sample was positive for the molecular swab but only 61% for the rapid swab; After 11 days, the molecular positivity was still 86% but decreased to 16% at antigen. Time is not the only factor affecting the sensitivity of the antigen smear: on average, the tests showed their ability to correctly detect positivity in 80% of symptomatic patients and 50% of symptomatic patients; Similar differences were found between vaccinated and unvaccinated. On the basis of these data, which depict a low efficacy of infection in the first days of infection, the researchers wanted to check how much the efficacy of the test would be improved if it was repeated twice: it was shown that by performing two smears at a distance of two days, an overall sensitivity of the test of 85% is reached. With swabs performed on two consecutive days, the effectiveness is 81%. “This finding suggests that symptomatic subjects with an initial negative antigen test should retest after a day or two,” the researchers concluded.
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