Beware of antigen tests: four reasons why they can give a false result

Antigen tests to detect a Covid-19 infection are already part of the daily life of society. The price of the tests in private clinics or the two days of waiting that a PCR entails have promoted the commercialization of these rapid tests, which is why hundreds of Spaniards resort to this test.

However, its widespread use does not imply that its reliability is the same as that of a PCR. In fact, antigen tests were only 58% accurate for people who had no symptoms, or 72% accuracy for those who did, based on a Cochrane review of more than 24,000 samples. In addition, these data correspond to the period prior to the discovery of the Omicron variant, so right now the ability of this test to detect it is unknown.

On the contrary, despite the difference in waiting time, a PCR yields drastically more reliable results, reaching an accuracy of 98%.

However, with the arrival of the Christmas holidays, many people will be in contact with their older relatives and, therefore, with a population at high risk of contagion, so many will be forced to resort to these tests, regardless its reliability. For this reason, it is vital to attend to the four factors that can falsify the results of an antigen test purchased in a pharmacy, according to Insider.

The tests have an expiration date

Out of fear of a possible shortage, many people purchased a large number of tests months ago. However, these tests have an expiration date, so cannot be used if it has been exceeded.

These kits of between 6 and 14 euros usually indicate on the box itself both the date of manufacture and the expiration date, so that the user can find them at a glance, it is always convenient to check this information before proceeding to use.

It should be noted that the expiration date of most of these tests tends to oscillate between four and six months. Anyone taking an expired test should assume that the result will be wrong.

Temperature affects the result

Both the temperature at which the test is kept and the temperature at which it is carried out will alter the test result, since the liquid that must be mixed with the extracted sample is extremely delicate and extreme temperatures can corrupt it.

“Don’t do rapid tests in very cold weather, that can decrease their effectiveness,” said Michael Mina, chief science officer at eMed, one of the companies that makes these kits. “You should do it at room temperature as much as possible”.

These temperatures vary between manufacturers, so, as with the expiration date, the temperature at which it should be stored will be indicated on the box itself or on the package insert. As a general rule, these kits should be stored between 2 and 30 degrees, although to use it, make sure that all the components are at a medium temperature.

Food and drink alter the test

At the moment, in Spain, there is only one authorized test that is performed orally and not nasal: Rapid COVID-19 Antigen Test / Hangzhou Alltest biotech Co. However, these may increase in the near future, so it must be taken into account that the food or drinks ingested will also affect the test.

Products such as coffee, fruit juices or any acidic food or drink, compromise the integrity of the test. However, at the very least, these types of products, including Coca-Cola, cause a false positive, rather than fail to identify the infection.

On the other hand, experts also do not recommend brushing your teeth or rinsing with mouthwashes. Users should not perform these acts, as well as should not eat or drink, during the 30 minutes before the test, at least.

It is unknown if they detect Omicron

At the moment, it is too early for the health scientific community to know the ability of these tests to detect the Omicron variant, so in many cases an infected person could give a negative result.