Covid: how to self-test and reduce the risk of a false negative result

When performing an antigen self-test to diagnose or rule out a suspected case of Covid-19, the most important step is to carefully read the instruction manual and strictly follow the manufacturer’s guidelines.

In general terms, these kits consist of three main items:

• The swab, that flexible rod that goes in the nostrils and in the back of the mouth;
• The tube, container that brings a liquid solution;
• The testing device, which receives the liquid and gives the result.

This exam is indicated when the person has symptoms suggestive of Covid, such as cough, sniffs, sore throat and fever.

The ideal is to take the test from the third day after the onset of discomfort, when the amount of virus in the body is high enough to be detected by the device.

Before that, the self-test can even be done, but a negative result cannot be taken to fire and iron – even more so if the symptoms persist or worsen.

The gathering

The steps may vary according to each manufacturer, but in general, the first guideline before starting the process is to wash your hands well, to avoid cross-contamination and a false positive result.

The next step is to take the swab and rub it about ten times on the inside walls of both nostrils — some brands also suggest putting the flexible swab in the mouth, deep inside the tongue.

Physician Evaldo Stanislau de Araújo, director of the São Paulo Society of Infectious Diseases, reports that many people are distressed by this type of collection, because they think it is necessary to deepen the swab too much, until it touches the throat.

“You have to lose that fear. At the beginning of the pandemic, every Covid test really required introducing the rod back there, in the nasopharynx. Some even joked that the swab almost poked the brain”, he recalls.

“Current tests generally do not require such a deep collection. It is important to pass the swab on the nasal wall. It does not have to go to the bottom, but it cannot be only on the tip of the nose”, adds the doctor, who also works at the Hospital. of the São Paulo Clinics.

In other words, passing the nose swab isn’t very pleasant, but it can’t cause pain or bleeding.

Infectologist Valéria Paes, from Hospital Sírio-Libanês in Brasília, suggests that the self-test be carried out in a well-ventilated place and without other individuals around.

“Often, the passage of the swab causes a certain irritation, and the person has attacks of coughing and sneezing. If he has Covid, he can end up transmitting the virus”, he warns.

the mix

The next step involves taking the swab and rubbing the tip of it in the liquid solution that comes in the plastic test tube.

Generally, manufacturers ask the user to make 10 to 15 circular movements.

The goal here is to mix the material that was collected from the nose with the liquid.

Then, just close the test tube with the cap and drip the stipulated amount of drops into the test device.

This object, usually made of plastic and rectangular in shape, has a specific hole into which the drops of solution must be dripped.

The result

After that, it is necessary to wait between 10 and 20 minutes for the result to appear.

What the testing device does is react to the presence of specific coronavirus proteins (or antigens).

If these antigens are actually present in the collected sample, two horizontal ribbons will appear on the device.

Now, if the person does not have Covid, only one tape will be visible.

It is very important that everyone knows how to interpret these results, as well as possible variations of them.

“The first thing is that the first of the risquinhos must appear in 100% of the reactions. If it is not there, you can discard the test because it has some defect”, points out Araújo.

“Secondly, the scratch needs to go from one end of the display to the other. If it is interrupted or appears incomplete, this also indicates that the exam is not working as expected”, he adds.

Another common question: what if the second line of the test is lighter, almost transparent? This represents a positive result for Covid, say experts.

“It doesn’t matter the intensity. If the second trace appears, even if it is very light, it means that there is coronavirus there”, answers Araújo.

“And it is worth remembering that the test result may take between 10 and 20 minutes to appear”, highlights Paes.

Mark on the clock the moment you drip the drops on the device, and wait the stipulated time to be sure of the result — often, the second tape only appears in the final minutes.

What to do after the result

If the test is negative and you continue to have symptoms, it is worth repeating the test again after two or three days to be more certain of the diagnosis.

Now, if the result is really positive, there are a number of actions that can be taken.

The first involves notifying the case to the health post closest to your home. This helps keep the pandemic numbers up to date and opens up the possibility of doing a PCR, a test whose results are even more reliable.

It is also important to remain in isolation and with minimal contact with other people. This way, you avoid transmitting the coronavirus further, eventually even to individuals who are at greater risk of developing the severe forms of the disease.

In this report by BBC News Brasil, you find out how many days of isolation are indicated, according to a series of criteria.

Finally, if you have Covid and need to take the third or fourth dose of the vaccine, it is worth waiting 30 days from the onset of symptoms to complete the immunization.

In this other article, you understand the reasons behind this recommendation and how to expand protection against Covid as much as possible.