Homemade antigenic swabs are reliable enough to detect SarsCoV2 positivity, however their maximum effectiveness is concentrated between the third and fourth day after the onset of symptoms. Therefore, if symptomatic, after a negative swab it may be advisable to perform a second test waiting a day or two, to definitively exclude the infection. This is the data that emerges from a study coordinated by the American Centers for Disease Control and Prevention and published in Jama Internal Medicine.
Covid, rapid swabs accused due to Omicron: too many positives escape
by Valentina Arcovio
The research considered 225 American adults and children whose positivity to SarsCoV2 infection was confirmed by the molecular swab and who also spontaneously underwent repeated tests at home. On average, the domestic swab was much less effective than the molecular one, however the difference between the two tests narrowed if the home-made swab was used in the right time window: between 3 and 4 days, in fact, the capacity of the antigenic swab to correctly detect positivity reached 77%.
Effectiveness decreases over time
The effectiveness, however, was found to drop rapidly: in the tests carried out by the researchers, six days after the onset of symptoms, 86% of the sample was positive for the molecular swab but only 61% for the rapid one; after 11 days, the molecular positivity was still 86% but dropped to 16% at the antigenic one. Time is not the only factor influencing the sensitivity of the antigenic swab: on average, the tests revealed its ability to correctly detect positivity in 80% of symptomatic patients and in 50% of asymptomatic patients; similar differences were found between vaccinated and unvaccinated.
Careless in the first few days of infection
On the basis of these data, which photograph a reduced effectiveness of the infection in the very first days of the infection, the researchers wanted to verify how much the effectiveness of the test improves if it is repeated twice: it emerged that by carrying out two swabs two days apart an overall test sensitivity of 85% is achieved. With swabs carried out on two consecutive days the effectiveness is 81%.
“This finding suggests that symptomatic people with an initial negative antigen test should re-test after a day or two,” the researchers conclude.