How to diagnose COVID-19?

Coronavirus diagnostic tests

Types of tests diagnose coronavirus to detect COVID-19

In the past we have suffered a great shortage of coronavirus diagnostic tests, fortunately this fact has changed and there are now several types of tests for infection screening in people. These tests are not scarce because they are carried out in the laboratory and do not require import. Below, we will describe some of the types of tests that exist, their reliability, when each of them should be done, results and a brief comparison that will help you choose the most convenient test as your case may be.


PCR diagnostic test

This is the most reliable test for coronavirus detection, you can find the virus at any stage.


Serology diagnostic test

This test is used to detect if you have passed the disease and if it generated antibodies (IgG/IgM).


Antigens diagnostic test

The fastest test so far is only recommended between the 1st and 5th day of contagion or symptoms.


PCR test to detect COVID-19

What is a PCR test?

It is a coronavirus diagnostic test that detects some of the genetic material of a pathogen.It is being used to diagnose whether or not a person is infected with covid 19 coronavirus, by exudating the nose and mouth to avoid errors and increase the amount of sample tested. This test is analyzed by molecular biology services in an accredited laboratory.

How does the PCR test detect the virus?

Using coronavirus diagnostic tests, the PCR test locates and amplifies a fragment of genetic material that in the case of coronavirus is an RNA molecule. If, after microbiological analysis, the test detects RNA of the virus, the result is positive, if the technique does not detect the genetic material of the virus, the result is negative. When there is a major clinical suspicion another test should be performed to ensure that the patient is not infected with the virus.

Scientific process to detect the virus with a PCR test


A smear is taken from the inside of the patient's nose or throat and the sample is taken for laboratory testing.



RNA is removed from the virus and purified. An enzyme called reverse transcriptase converts RNA into DNA.



It is mixed with barley, and an enzyme that synthesizes DNA, which produce millions of copies of viral DNA.



Fluorescent dye molecules join DNA during copying, producing more light, confirming the presence of the virus.


Serological test to detect COVID-19

test test serology coronavirus
What is a Blood Serology test?

Serology is a test that analyzes the body's serums using enzyme-linked immunoabsorption (ELISA). The parts of the blood that after being clotted still maintain their liquid state.

How does the SERology test (IgG/IgM) detect the COVID-19 virus?

  • It analyzes the antibodies in the body and allows you to see if an infection occurs. If a person has an infection, the body will generate more antibodies.
  • Serology will detect these new antibodies that are being generated and determine the type of infection that is being suffered.
  • This way you can know if you have previously suffered from any infection, if you are immune to certain microorganisms.

Serology is done through a blood draw and blood serum testing. Blood draw must be carried out by personnel empowered to do so, in duly approved media.

When to get a blood serology test for the coronavirus?

It is recommended to perform it from the 4th or 5th day of possible contagion and without deadline from this, since it can indicate the presence of antibodies after the disease, even without having had symptoms.

IgM occurs in primary states of COVID-19 coronavirus infection, immunoglobulin M is found mainly in the blood and lymph fluid; this is the first antibody the body manufactures to fight a new infection, it is a classic marker of immune response. It can be detected on the 5th-7th day. It is considered important in the following cases:

People who come with evolved disease with more than 7 days of symptoms, people with symptoms and negative PCR. Study of people in contact and with difficulties in confinement, to see if they have immunity and therefore low risk of infection.

What results does a complete Serology test offer us?

IgM levels: begin to increase from the 5th day of the onset of symptoms (between the 3rd and – 7th day) to days 8 to 14 and then are maintained between days 15 to 21.

IgG levels will determine whether or not we have antibodies to the disease, so we can find out if we have suffered it, even though we are asymptomatic and if we are protected by antibodies from another possible contagion.

The result of a serological test is validated by a medical physician in the laboratory and must be interpreted by another medical physician, who will also presumably be responsible for performing the certificate of immunity if appropriate and/or giving us the indications of treatment if necessary.

coronavirus diagnostic test


Antigen test to detect COVID-19

What is an antigen test?

The Antigen Test is a rapid coronavirus diagnostic test that is certified to detect SARS-CoV-2 disease, requires no instrumentation and provides results in 30 minutes, making it a valuable tool for mass testing in decentralized environments. It is also an accessible and scalable option. It has 93.3% reliability and 99.4% sensitivity.

How does the virus detect the Antigen test?

The test reacts chemically to the presence of the virus between the 1st and 5th day of infection, after the 6th day it loses reliability. A negative result does not rule out Covid-19 infection. If you want, it can be confirmed with a PCR or Serological test.

Difference between tests: PCR, Serology and Antigens

Next, we leave you a graph containing the differences between PCR, Serology and Antigen tests, with it you can get an idea of how long the results take for each, the laboratory, sample type, the type of information you obtain and the reliability; with this you can come to a conclusion or make a comparison to know which test benefits you the most depending on your case.

coronavirus diagnostic tests

What test is done to detect the COVID-19 virus and diagnose the disease?

In short, according to your needs there is proof to do so:

PCR is recommended when:

It is recommended after the 3rd and until the 21st day of suspected infection or symptoms. Most airlines also recommend it for travel.

Serology is recommended when:

Between day 10 and 14 of the possible contagion and with no deadline from it, as it may indicate the presence of antibodies after the disease has passed.

Antigen is recommended when:

It is recommended only between the 1st and 5th day from suspected infection or close contact with a positive or symptoms. After the 7th day he loses reliability.

In the graph below you can see the period where the tests are most sensitive.


If you want to get any of our tests, you can go without an appointment to our center. See the menu at the top for more pricing and timetable information.

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Diciembre 2021

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Avda. de San Luis 164, abierto para realizar pruebas de Covid. Resultados mismo día.