What is the right day to do each test and avoid false negatives

With the rise in the number of cases of covid-19, doubts about how to take a test to know if you have the virus also appeared again. In order not to make mistakes and bet the chips on a false negative, keep an eye on the exam window, whether you have symptoms or not.

Remembering that there are two situations that ask for a test to diagnose or rule out covid-19.

The first of these is when you have two or more of the typical symptoms of the disease, as shown in the list below:

  • Fever
  • Cough
  • Tiredness
  • Loss of taste or smell
  • Sore throat
  • Headache
  • Body ache
  • Diarrhea
  • Skin or eye irritation

In this case, the ideal is to stay in isolation, even before taking the test or if you cannot perform the exam due to lack of availability.

The second is when you have had close contact with someone who is suspected or has already been diagnosed with covid-19.

If you have not shown symptoms, wear a quality mask and avoid contact with members of risk groups for 10 days, guides infectious disease specialist Moacyr Silva Junior, from the Hospital Infection Control Service at Hospital Israelita Albert Einstein, in São Paulo.

When each type of test should be done

PCR: this test is considered the most efficient to diagnose covid-19, because it detects genetic material from the virus in the sample in real time. Therefore, it is indicated for those who need to be sure that they are not contaminated and have not yet shown symptoms. However, it is a more expensive test, the result of which is sent by a specialized laboratory – which can take up to 72 hours.

The collection, through nasal swab, must be done up to eight days after the first symptom. In this case, the accuracy is more than 90% accurate, with very rare false positives.

When there were no symptoms, the ideal is to count exactly five days after the last contact with the person who discovered to be infected, says Alberto Chebabo, director of the Clementino Fraga Filho Hospital, at UFRJ (Federal University of Rio de Janeiro) and medical relationship manager from Dasa. “Consider this meeting as day zero.”

If you do not respect this window, the risk of a false negative is greater.

In cases of flu syndrome (GS) or severe acute respiratory syndrome (SARS) in the acute phase of infection, collection should be performed as early as possible. For hospitalized critically ill patients, the collection can be done up to the 14th day of the onset of symptoms.

Antigen: also called “rapid test”, it can be done directly in health units or pharmacies, using a nasal swab. It is indicated for those who had symptoms, as it detects the virus when it is contaminating.

To avoid a false negative, test in the acute phase of the disease, that is, from the 1st to the 7th day in people with symptoms.

Those who did not have symptoms, but had contact with contaminated people, can take the test from the 5th day of the meeting. As the test does not give a false positive, it serves to confirm the infection — but not to rule it out.

The advantage here is that the antigen test is ready in up to 30 minutes. The downside is that if the viral load is low, the risk of a false negative is high.

self test: it works in the same way as the antigen test, the difference is that you buy it at the pharmacy and do it at home, on your own. The so-called immunochromatographic method, which indicates the presence or absence of an antigen by means of color, comes in a format that resembles a pregnancy test.

It is the cheapest test available. As it is done by a lay person, it is essential that all instructions are followed. The result appears in a few minutes.

It was negative, what to do?

Moacyr Silva reinforces that a false positive in the tests is very rare. So if the result is positive in the antigen test, you do not need to do a PCR later to confirm the diagnosis.

When the self-test or antigen test is negative and there are symptoms, the right thing is to complement with PCR or maintain isolation anyway.

If the PCR was negative, you may have the flu, as we are experiencing an epidemic of influenza, sinusitis, or other respiratory illness.

With information from texts published on 08/13/22.

About Jenni Smith

She's our PC girl, so anything is up to her. She is also responsible for the videos of Play Crazy Game, as well as giving a leg in the news.

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