In the quest to find out if you are infected and protect co-workers and family members, rapid antigen tests are playing a central role. Companies and ordinary people have been looking for this solution to move closer to pre-pandemic normality. They help to detect infections early and stop chains of contagion.
Many citizens prefer to undergo a test before visiting relatives in nursing homes or special care homes, or before attending private meetings. However, the resulting sense of security can prove to be misleading, especially at a time when the ômicron variant is dominant.
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At the end of last year, the FDA – the regulatory agency of the United States – warned that rapid antigen tests are less sensitive to the omicron than the previous variants, and could result in false negatives.
Even before the FDA declaration, there were already doubts about the reliability of the rapid tests. Researchers took a closer look at a large number of the available antigen tests: 20% were disqualified because they did not respond to even a large viral load. According to one study, 26 of the 122 tests examined did not even meet the minimum requirement of 75% sensitivity.
This amount is defined by the Ct index, referring to the number of times a sample needs to be reproduced in the laboratory until the genetic material of the virus is detectable. The lower this index, the higher the viral load: a Ct of 25 or less indicates that the carrier is extremely contagious; from 25 to 30, the viral load is high; between 30 and 36, moderate.
For coronavirus testing, secretions are collected from the airways, with a swab from the nose and/or throat. If the sample contains a small amount of virus, because the patient is only at the beginning or end of the infection, or if he has ingested a lot of fluid before collection, the express tests are not very reliable.
Therefore, in the case of a positive result, a polymerase chain reaction (PCR) test should be performed immediately afterwards, which is much safer.
All this, however, does not mean that quick tests are useless. They often help to contain the pandemic, helping to detect infections very quickly, thus breaking the chain of contagion, especially when it comes to carriers with a high viral load.
However, even those who test negative can be infected and contaminate others. Two days before the first symptoms, covid-19 patients are already contagious. At this stage – or in the case of an asymptomatic condition – antigen tests are not entirely safe. That’s why you shouldn’t rely too much on your results, relying on a deceptive sense of security.
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