Test types


RT-PCR test, baseline technique for the diagnosis of covid-19


The RT-PCR test is the baseline technique used to carry out the microbiological diagnose of covid-19.

With this test we can explicitly confirm that the infection has been caused by the SARS-CoV-2 and not by any other virus or bacteria.


Efficiency in RT-PCR testing

The principle of the RT-PCR test is based on the amplification of certain genetic material sequences of the SARS-CoV-2 virus. This is the reason why it is the most precise and effective test to diagnose covid-19. It can differentiate two very close microorganisms evolutionally speaking, so it can undoubtedly identify the SARS-CoV-2 virus when there are other types of coronavirus.

RT-PCR test result

It is a complex technique, a very sensitive and specific one, which requires qualified professionals to carry it out and interpret results. Nevertheless, ATimmunolab in Madrid is ready to carry out diagnostic tests and deliver results within 24 – 48 hours (work days) after receiving samples.

Sample drawing: RT-PCR test

At the initial stage of the infection, the SARS-CoV-2 virus lies in the larynx. That’s why respiratory samples are needed to carry out the RT-PCR test: nasopharyngeal exudate, oropharyngeal exudate or sputum samples, mainly.
This test must be carried out during the first days of the infection and a positive result confirms that the patient is infected with SARS-CoV-2.

Rapid antibody detection test (TDR): serological test

In order to reduce waiting time and increase the diagnostic capability of the National Health System, rapid antibody detection tests are also used, known as serological tests, to diagnose covid-19.

Principles of the rapid antibody detection test

The principle of this test is based on the detection of total antibodies and/or IgM and IgG by the joint of antigen-antibody, the joint of specific antibodies for proteins from the virus, which are present in the samples of the infected patients, with the antigens, antigen proteins of the SARS-CoV-2 virus, present in the kit.

The department of health has established using these rapid detection tests first and foremost in nursing homes and public health centres in order to rapidly detect possible outbreaks. Even so, the baseline technique is the RT-PCR, so when the rapid antibody detection test carried out in patients with a high clinical suspicion is negative, a RT-PCR test must be done to assure the diagnosis.

Advantages of the rapid antibody detection tests

They are quick. Results are available around 15 minutes after the sample has been taken.

They are easy to use. These tests can detect antibodies to the SARS-CoV-2 on blood samples normally taken by finger prick.

They are a complement to establishing the evolution of the infection in confirmed cases. Apart from the microbiological diagnosis carried out by the RT-PCR test, the patient must be continually monitored in order to evaluate different parameters. As in any infection case, when the virus enters the body, multiple defence strategies are activated to eliminate the infection. One of these strategies is producing antibodies: IgM and IgG.

Detecting these antibodies (immunoglobulin) makes it possible to confirm and obtain information about the progression of the disease. Although there is great variability among people, 5 to 7 days after the infection starts, IgM antibodies can be detected (although they are better detected 8 to 14 days after the infection begins), then they peak and later on they start to fall. IgG are detected mainly 15 to 21 days after the beginning of the infection and they remain longer in the body, although how long they will stay is not clear yet.

Disadvantages of rapid antibody detection tests

They can give incorrect results: False negatives

Most patients develop antibodies between the 6th and the 15th day after the beginning of the infection, so it is important not to carry out this test before this time period as it could lead to a wrong diagnosis. Also, there are a lot of trading houses with different qualities so it is important that tests are as specific and sensitive as possible to avoid incorrect results.




Other techniques to detect coronavirus

Apart from the RT-PCR test and the rapid detection test (TDR) there are other serological techniques that can help monitor the patient and the course of the covid-19.

Among these tests, normally used in hospitals and diagnostic centres, we highlight rapid viral antigens detection tests and the ELISA technique:

Rapid viral antigens detection test

Unlike RT-PCR, viral antigens tests do not identify the genetic material of the virus but proteins (antigens) that are present in the respiratory samples of nasopharyngeal exudate and oropharyngeal exudate.

There are several trading houses with different qualities and according to information published by the WHO, sensitivity of these tests ranges from 34% and 80%

For now, nor the WHO or the department of health recommend these tests to diagnose covid-19.

ELISA technique

It is a specific and very sensitive technique that detects antibodies and makes it possible to automatically take a high number of samples, which can be quantitative.

It must be carried out between 1 and 2 weeks after the first symptoms appear and it complements the RT-PCR.

It is especially important when a confirmed case by a RT-PCR test shows a negative result when the test has been repeated. As we pointed out, the results of these antibodies can guide professionals and help them establish the phase of the disease the patient is in.

For example, a patient with a negative PCR, low levels of IgM and IgG levels suggests that the patient is in the final phase of the disease or has overcome it.

ATimmunolab, experts in RT-PCR
tests to diagnose covid-19

Natural course of covid-19

Transmitted diseases have different phases. Regarding covid-19, both an early diagnosis and controlling how the patient’s condition affects each phase are decisive to manage the evolution of the disease.

Pre-pathogenic period of covid-19

It is the period when people live with the virus (they share the same environment) but they do not present symptoms or changes in their body.

Incubation time (pathogenic)

Microorganisms easily reproduce in the body but the person does not have any symptoms.

Clinical period

The symptoms and clinical signs of the patient determine this phase.

Duration of phases regarding covid-19

There is great variability among the people infected, but according to the latest updates published by the WHO and the department of health (May 2020), right now it is considered that the average incubation period is 5-6 days with a range of 1-14 days, and transmission of the infection generally starts 2-3 before the first symptoms. It is not known if the intensity of the transmission is the same in asymptomatic people and in people with symptoms.

As for duration of the disease, it is estimated that in patients with mild-moderate symptoms the average time from the first symptoms until they recover from the disease is 2 weeks, whereas in seriously ill patients it is from 3 to 6 weeks.


Depending on the intensity of the symptoms observed until now, 4 groups have been considered to classify patients infected with SARS-CoV-2:

  • Asymptomatic (they are ill but with no noticeable symptoms)
  • Mild to moderate
  • Serious illness
  • Critical illness

Symptoms associated with covid-19

According to data collected so far, 80% of ill people present mild-moderate symptoms or are asymptomatic, 15% need hospital admission and 5% need intensive care.

Although symptoms can be very different, a great part of the people infected that present symptoms get fever and dry cough in Spain (68,7% and 68,1% respectively), among some other symptoms.

Losing sense of taste and smell is another symptom recently associated to covid-19, but it is also very important because even though not all patients show these symptoms and they are not exclusive to this disease, they actually are an indicative of covid-19.

Risk factors for covid-19

At present, the cause that makes some patients become seriously ill is not known.

However, suffering from any of the following illnesses increases the risk of finding complications for patients with covid-19:

  • High blood pressure
  • Cardiovascular diseases
  • Chronic obstructive pulmonary disease
  • Diabetes mellitus
  • Cancer
  • Immunodeficiency
  • Chronic kidney failure

People over the age of 60 and pregnant women are also considered risk groups.

There is not much information regarding pregnant women but it occurs naturally that they experiment physiological and immunological changes that make them more vulnerable to develop any infection. Thus and as a precaution, they are included in groups at greater risk.

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RT-PCR test service for clinics

Quick and effective diagnosis for covid-19. Comprehensive service for hospital centres with private access to test results.