COVID: PCR or antigens, which test is better?

More than two years after the start of the pandemic of COVID-19, there are still doubts about which of the tests to detect the virus is the most appropriate.

These tests are used to know in time that you caught the coronavirus and thus isolate yourself and avoid infecting other people, but also to monitor your health in order to receive timely medical attention.

There are at least three types of tests to detect COVID-19 and their application and functionality are different according to the symptoms and characteristics of each case.

Below we list the types of COVID tests that exist and the characteristics of each one so you can determine with your doctor which of all to perform.

What COVID tests are there and what are the differences?

PCR test

According to the World Health Organization (WHO), these diagnostic tests detect the genetic material of the virus. That is to say, detect an ongoing SARS-Cov-2 infection.

This is done by a health professional taking a sample with a swab in the respiratory tract (nose or mouth) and it allows to distinguish even asymptomatic cases.

Detection is done at the molecular level in a specialized medical laboratory.

Results are accurate and are released within 24 to 48 hours, depending on the circumstances. There are laboratories where they take up to 72 hours to deliver them.

This test is the most reliable and its application is suggested from 1 to 7 from the onset of symptoms according to the Jalisco Health Secretariat.

Antigen test (or rapid test)

This test, according to the WHO, detects viral proteins (antigens) and also detects an ongoing SARS-Cov-2 infection, which is the virus that causes the coronavirus.

The sample is also nasopharyngeal, since they can be via the nose or mouth

Epidemiologist Michael Mina points out that although these tests are not as good as PCR, they have other advantages such as being able to be applied at home, being faster and cheaper.

It indicates that a positive result is reliable, but a negative result may indicate that there is not enough viral load yet to be tested.. “A negative does not ensure that one is free of the disease and it is advisable to repeat the test if there is suspicion of contagion.”

The immunologist and researcher Jesús Abraham Simón points out that “the antigen test has a low sensitivity and there is a high percentage of false negatives “.

The Jalisco Health Secretariat recommends that this test be run 3 to 7 days after the onset of symptoms.

The results can be known from 15 minutes to two hours. It can be done at home or it is also applied in laboratories or even in offices attached to pharmacies.

The more viral load present, the darker the second line will appear that the result marks a positive.

Specialists recommend not trusting the first antigen test if you have symptoms, isolate yourself and take another test, either for antigens or PCR in the following days.

According to the Centers for Disease Control and Prevention (CDC), these tests work best in people who are within the first three days of having symptoms started, but the sensitivity of the test is very low in people who do not have symptoms. .

If it is positive, you have COVID, the CDC highlights, but if it is negative it has the possibility of being false in up to 40% of cases, so it is not ruled out that you have COVID.

Antibody or serological tests

According to the WHO, it detects antibodies against the virus. That is to say, quantifies the immune response against infection.

Does not help timely diagnosis.

Blood samples are used and it only allows distinguishing who has been infected previously, but does not diagnose an ongoing infection.

Patients with COVID-19 generate antibodies within 10 to 30 days of the onset of symptoms. However, antibodies can also be detected in vaccinated people.

It is recommended to be done from the seventh day of the first symptom and the results are in 15 minutes.



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