COVID-19: how to know if the negative result of a self-test is reliable

COVID-19 self-detection tests were a step forward in managing the pandemic at a global level. In the midst of the explosion of infections by the Ómicron variant and with the testing centers collapsed in December last year, its use spread throughout the world. In Argentina, they were approved by ANMAT in January and authorized for sale in pharmacies throughout the country.

A home test is a practical solution to the usual symptoms of coronavirus, such as fever, sore throat, cough or runny nose. After wiping a swab through the nose and waiting for about 15 minutes, the result is obtained in the comfort of home.

However, many are wondering, is a negative result reliable? Or maybe the test didn't detect the virus? The effectiveness of the test will determine whether a new PCR test is necessary or if, despite the symptoms, one can return to work and reunite with others.

The rapid antigen test detects the nucleocapsid protein of the virus. Such a protein is found on the surface of the structure of the virus, so the presence of the virus is detected much faster. From there, its result lies in a few minutes. In contrast, the PCR test looks for the presence of genetic material from the virus. However, a negative home test result does not guarantee the absence of the SARS-CoV-2 virus.

“It's possible that the virus is fighting your immune system in your body,” said Michael Mina, American doctor and chief scientific officer of eMed, an online help firm for rapid testing. “If the test is negative and you have symptoms, don't take the negative for granted. What needs to be taken for granted is that the virus has not had a chance to develop. The symptoms may mean that your immune system is only giving a very early warning,” the doctor told the New York Times.

This means that the harder the immune system works to put a barrier to SARS-CoV-2, especially if it is reinforced by the antibodies in the vaccine, the more likely it is to get a negative result with a home test, even if the person is infected.

A positive result in the early days of coronavirus symptoms means that COVID-19 infection is almost certain. But if the result is negative and the symptoms continue, precautions should be taken, wearing a mask and avoiding close contact with others.

According to experts, it is advisable to wait 48 hours and repeat it. If the result is still negative, but the symptoms persist or worsen, a new test should be done on the fourth day. In these cases, the ideal is to go to a test site or health center for a PCR test, which can detect the presence of SARS-CoV-2 earlier than a home test.

In this regard, when approved in Argentina, the ANMAT noted that “it is necessary to bear in mind that if the person has no symptoms or if the viral load is low (which may occur during the initial or final days of infection) SARS-CoV-2 may not be detected by the test, so a negative result does not rule out infection.”

“If symptoms persist and tests continue to be negative, the person's chances of being infected with COVID-19 and being contagious are greatly reduced,” said Robert Wachter, head of the Department of Medicine at the University of California at San Francisco. “But most likely that day you should wear a mask because you have something.”

So, while the negative result of a home test is inconclusive and a new test is necessary if symptoms persist, COVID-19 home tests are effective in determining whether a person is highly contagious, since the result will be negative if the viral load is not so high enough to infect many people.

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