Understand how it's done and the results of the different tests for Covid-19

Many people can have COVID-19 and be asymptomatic, meaning they do not show any symptoms. To really know if a person is infected, there is only one way: to get tested for the diagnosis.

But the amount of tests available has raised many doubts. Therefore, we brought some clarifications about each type of exam. Check out:


RT-PCR (  reverse-transcriptase polymerase chain reaction ) is considered the best in the diagnosis of COVID-19. The test is preferably performed by scraping the nasopharynx. After collection, it is analyzed whether there is detection of SARS-CoV-2 RNA in the analyzed sample.

If there is SARS-CoV-2 genetic material in the sample, specific probes detect its presence and emit a signal, which is captured by the equipment and translated into a positive result. With this result, the suspicion of COVID-19 is confirmed.

To perform the procedure it is necessary to have the request of your doctor. The collection can be done from the 3rd day after the onset of symptoms and up to the 10th day, because at the end of this period, the amount of RNA tends to decrease. In other words, the RT-PCR test identifies the virus during the period it is active in the body, making it possible to apply the appropriate medical conduct: hospitalization, social isolation or other procedure relevant to the case in question.

There are several methodologies and protocols for performing RT-PCR, so the results may vary from one laboratory to another.


Serology testing checks the body’s immune response to the virus. This is done by detecting IgA, IgM and IgG antibodies in people who have been exposed to SARS-CoV-2. In this case, the exam is performed from the patient’s blood sample.

For the test to have greater sensitivity, it is recommended that it be performed at least 10 days after the onset of symptoms. This is due to the fact that the production of antibodies in the body only occurs after a minimal period of exposure to the virus.

Performing the serology test outside of the indicated period may result in a false negative result. Therefore, to perform the examination, a medical request is required. In case of negative result, a new collection may be necessary, at the physician’s discretion. It is also important to emphasize that not all people who have SARS-COV-2 infection develop detectable antibodies by the available methodologies, especially those who present with mild symptoms or do not present any symptoms. Thus, there may be negative serological results even in people who have had COVID-19 confirmed by PCR.

quick tests

Two types of rapid tests are available on the market: antigen (which detect Covid19 proteins in the active phase of the infection) and antibody (which identify the body’s immune response to the virus). The advantage of these tests would be to obtain quick results for the decision to proceed.

However, most existing rapid tests have very low sensitivity and specificity compared to other methodologies. The Ministry of Health points out that rapid tests have an error rate of 75% for negative results, which can generate insecurity and uncertainty to interpret a negative result and determine whether the patient in question needs to maintain social isolation or not.

As the rapid test does not have the same sensitivity as other methods, it is important to have the guidance and monitoring of a doctor. Rapid tests for COVID-19 are similar to pharmacy tests for pregnancy. In the case of the test for COVID-19, a nitrocellulose slide (a kind of paper) is used that reacts with the sample and presents a visual indication if positive.

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