PHONE: +52 442 6889133

COVID tests: which one should I get? how many exist?

» ProSalud

Know the differences between PCR, antigen and serological tests and which would be the most indicated according to the circumstances.

Which Of the COVID tests most effective to know if you were infected? When is the best time to do it to avoid a bad result? false negative?

El Dr. Oscar Tamez, pediatric infectious disease specialist and member of the COVID team TecSalud, explains that currently there are 3 tests to know if you are or were infected with the SARS-CoV-2 virus.

The three most common options are:

– PCR test. the most effective to detect COVID with or without symptoms

– Antigen test: not very effective in detecting COVID without symptoms

– Serological test: to confirm if you have previously had COVID 

The specialist points out that these 3 options differ both methods sampling, Insights, And in results. Its use It will depend on the presence of symptoms and disease progression.

– Which test is best for me?

COVID tests: what are their differences and which one should I get?


PCR tests: the most effective to detect COVID with or without symptoms

It is a type of test molecular diagnosis. His name is RT PCR (reverse transcription polymerase chain reaction) and its objective is to detect the presence of genetic material of the SARS-CoV-2 virus in the patient.


It's the test with greater sensitivity and the one that offers the more accurate results.


It is relatively the most expensive and the one that can take the longest. As it looks for the presence of genes, it can test positive by finding dead or inactive traces of the virus.

Sample collection:

Nasopharyngeal (nostrils or mouth) through a hyssop to collect mucosal secretions that are then sent to the laboratory for molecular analysis.

What it detects

presence of RNA or ribonucleic acid from the SARS-CoV-2 virus.

Who is it recommended for? 

For people who suspect they are infected or who have been exposed to possible infection. It's just as efficient in presymptomatic, symptomatic or asymptomatic people.


Up to 94% to detect the disease.

Results delivery time

There is an estimated time of between 24 and 48 hours to be delivered to patients.

PCR tests are the most effective for detecting COVID-19.

Antigen tests: not very effective in detecting COVID without symptoms

They are diagnostic tests of the most common and quick to detect if a person is sick with COVID-19, since with them you can obtain results in less than an hour

However, they can also represent a risk due to their high degree of false positives.


Its speed and relatively low cost.


Su lack of effectiveness in detecting asymptomatic patients that can spread the virus.

Sample collection

Nasopharyngeal (nostrils or mouth) through a hyssop. The sample is diluted in a liquid medium that gives a colorimetric result, that detects or does not detect the presence of the virus.

What they detect

Hen/Stag proteins found on the surface of the virus.

Who is it recommended for?

Only for people with present symptoms.


In symptomatic people, between 70 and 90%. In asymptomatic people, false negatives may occur that range between 50 and 55%.

Results delivery time

De 40 minutes to an hour to get the results.

“A patient without symptoms does not benefit from an antigen test, they must take a PCR test; If the patient begins to have symptoms, the antigen test recovers this diagnostic performance and is almost on par with a PCR", says the specialist.

Serological tests: to confirm if you have previously had COVID

These tests still detectbodies in the blood of patients who have had COVID-19 infection in the last 3 months. They show results qualitative or quantitative.

However, according to the specialist, there is no official recommendation on when to take this test and Its use is individualized that responds to different purposes, for example, for immunocompromised patients or with multisystem inflammatory syndrome.


Simplicity and relatively low cost. Only indicate if you were infected in the last 3 months.


It is not useful to know if you currently have the disease. They are no longer being used because their performance may be poor.

Sample collection

The first method is through a drop of blood from a finger which is applied to a rapid colorimetric methodology test. It only tells you whether or not there is the presence of antibodies.

In the second, we obtain serum through a puncture in a vein, which is analyzed in a laboratory and gives the amount de antibodies present.

What they detect

Antibodies Immunoglobulin G (IgG) and Immunoglobulin M (IgM) that produced to combat infection.

Who is it recommended for?

For those who want to know if they suffered from COVID-19, for those who want to know if they have antibodies, and for those who need to know their amount of antibodies in the face of a medical need.


According to the doctor, a percentage of the population between 30 and 35% do not raise antibodies despite having had a documented infection.

Results delivery time

Between 5 and 10 minutes in quick testsIn laboratory tests, the time can vary from hours to days.

“A patient without symptoms does not benefit from an antigen test, they must take a PCR test.”

“At first there was a lot of talk about rapid tests to detect an acute condition with the detection of IgM, the antibody that would be elevated in an acute infection, but we have seen that with COVID-19 in a rapid test it does not give us that certainty diagnosis.

“That is, a absence of IgM it doesn't tell us anything and that's why we are using molecular diagnostic tests", adds the pediatric infectious disease specialist from TecSalud.

Which COVID test is best for me?

According to Dr. Tamez, PCR and antigen diagnostic tests allow us to know if a person is infected and has the active virus.

When to use PCR?

– When you suspect COVID-19 infection.

“Whether due to symptoms or close contact with an infected person, regardless of whether the patient has symptoms or not,” Explain. 

– Wait 2 to 5 days

Do not seek to apply it if you met a person confirmed or suspected of having COVID.

When to use the antigen test?

– Only when you have symptoms

The health specialist recommends its use in patients who present obvious symptoms of the disease.

“An antigen test to diagnose an asymptomatic patient is of no use,” comments the doctor.

– If you test negative and have symptoms, do a PCR

The health specialist suggests do a PCR when the patient has obvious symptoms and antigen test is negative.

The most effective COVID-19 diagnostic tests currently are PCR and antigen tests.

Isolate yourself if you cannot get a test

In the event that a person does not have access to a diagnostic test, Dr. Tamez suggests comply with isolation Minimum of 10 days in people with mild symptoms y 14 in case of more severe symptoms of COVID-19.

“If you decide to follow the route of not getting tested after having been in contact with a symptomatic person, it is important that you comply with this isolation of, at least, 10 to 14 days.

“Now, if you developed symptoms and you are not going to take your test, they should be minimal. 10 days from the onset of symptoms", said.

They wait for the saliva PCR

A future option, which is already being implemented in countries like the United States, is saliva PCR test, points out the doctor.

“It is quite comfortable, because whether it is with a cheek massage to obtain saliva or if the individual can spit, it avoids that issue of invasion.

“It would be very promising for the pediatric population. Here in Mexico an authorization has not been officially issued, but It is a matter of time before it is available and it will be very comfortable for the general population,” says the doctor.

“If you decide to follow the route of not getting tested after having been in contact with a symptomatic person, it is important that you comply with this isolation of, at least, 10 to 14 days.”


author avatar
Yair Ramirez
Currency / Currency
Open chat