Since the coronavirus pandemic reached our borders, we have not stopped talking about PCR (polymerase chain reaction), antibodies, IgG, IgM... The last one to be added to the list has been the test of antigens.But are we clear about what each result means if we should undergo a PCR, an antigen test or do a serology?
The most common symptoms of coronavirus defined by the World Health Organization (WHO) are fever, dry cough and fatigue.However, other less frequent symptoms are recognized, such as pain or discomfort, nasal congestion, headache, sore throat, lack of taste or smell, conjunctivitis, diarrhea, skin rashes and certain changes in the tongue. what has been called Covid Language.Once these symptoms appear, a doctor may order a PCR or antigen test to confirm or rule out the diagnosis.Now, in addition, antigen saliva tests are being proposed on an increasingly closer horizon as another way to detect the presence of the coronavirus in the body. Neutralizing Ab
Furthermore, since the omicron variant appeared, the symptoms are more similar to those of a regular cold.This new strain, which has produced unprecedented infections since the beginning of SARS-CoV2, appears to be somewhat milder, and the infection generally remains in the upper respiratory tract without affecting the lung.
PCR is a molecular technique that allows the detection of the virus genome, either from its RNA (ribonucleic acid) or its DNA (deoxyribonucleic acid).“In the case of the coronavirus we detect the RNA of the virus.That is, when the virus infects a person, it enters the cells of the nasopharynx and begins to replicate.Using a sample from the nasopharyngeal mucosa area, we try to detect the RNA of the virus,” Pilar Catalán, head of the Virology Section of the Microbiology Service at the Gregorio Marañón University Hospital, explains to CuídatePlus.
This complex technique, which must be performed in the laboratory, allows us to know if the person has had or has a coronavirus infection.And it can be detected both a few days before symptoms begin and a long time after they disappear.“The PCR test determines whether or not there is presence of the virus, not the amount of virus – viral load – that the patient has,” adds Daniel Carnevali, head of the Internal Medicine service at the Quirónsalud Madrid University Hospital.
The PCR can remain positive for several weeks and even a few months.But this does not have to mean that the virus is still active, but rather that fragments of the coronavirus continue to be found.“What rules is the patient's symptoms” and if the symptoms have disappeared, the person is considered to have been cured, explains Catalán.
Long-chain PCR has also been heard of recently.This is a test that has usually been used in other fields of Medicine, such as Genetics or Oncology, and that uses very long fragments of DNA.According to Alberto Delgado-Iribarren, head of the Microbiology Service at the San Carlos Clinical Hospital in Madrid, these techniques are more expensive and more complex than conventional PCRs with which the coronavirus is diagnosed, but they do not imply greater reliability in the result. ."Diagnosis by conventional PCR is just as reliable because it is performed at the strain characterization level."
Rafael Cantón Moreno, head of the Microbiology Service at the Ramón y Cajal University Hospital in Madrid, adds that "they are strategies that are used to amplify a wider region than is done with more conventional amplifications. This allows us to overlap some fragments with others and characterize some variants in a certain way. Long PCR is a diagnostic tool and is not exclusive to coronaviruses."
As the pandemic progresses, mutations of the original coronavirus appear, giving rise to variants.Several have already been identified: the British, the South African, the Brazilian and Omicron.Some people wonder if their PCR test will also tell them, in addition to whether it is positive or negative, if they have been infected with some of these new strains.Currently, PCR is already being used in the Microbiology laboratories of some Spanish hospitals - making the relevant technical modifications - to detect the British variant.
At the moment it is not done with all samples since it is a complex process."What needs to be told to the patient or the doctor treating them is whether the person has coronavirus or not," adds Cantón.And the fact of having been infected with one strain or another does not change either the treatment or the isolation and hygiene measures that must be taken.This information collected on the presence of mutations is, however, of interest for "epidemiological studies and to know how the different variants of the coronavirus are penetrating," adds Delgado-Iribarren.
Likewise, coronavirus genome sequencing studies are being carried out with some samples to continue increasing knowledge about this pathogen.
The last to arrive on the coronavirus scene in Spain have been the antigen tests.What are they?According to Delgado-Iribarren, antigen tests serve to detect “the presence of the virus in a very simple way and with very fast devices, like those used in pregnancy tests, etc.”
One of its advantages over PCR is that the results are obtained in about 15 minutes and it is not necessary to have a specialized laboratory.Speed allows us to advance decision-making regarding confinements and contact tracing.At first, antigen tests could only be performed in the healthcare setting, but for some time now they can be performed at home by performing a nasopharyngeal exudate with the swab they include.
Although antigen tests are cheaper than PCR, they are not as sensitive.However, they allow people with a high viral load to be diagnosed and, for now, with any variant.“It is a strategy that is being followed mostly in many countries because we are very interested in detecting those patients with a large amount of virus because they can be great contagions,” says Delgado-Iribarren.
PCR allows the coronavirus to be detected at any time during the infection.For their part, antigen tests are useful in the first moments after infection has occurred, that is, in the 5 to 7 days afterward, when there is a greater amount of virus.However, although at these times the effectiveness between both tests is similar, Delgado-Iribarren explains that the antigen test is mainly applied to people with symptoms.
In addition to at home, antigen tests are carried out in health centers and hospitals due to their great diagnostic capacity: “we are interested in ensuring that the emergency room does not have crowds.Its usefulness is very great.”In fact, moving the antigen test to Primary Care has, in a way, made it possible to decongest hospital laboratories, which have been largely focused on performing PCR since the pandemic began.
The use of antigen tests in asymptomatic patients is much less useful because the time in which they have been in contact with the coronavirus cannot be determined."It is useful if we can limit the infection. There it is convenient to use a PCR, since even if there are lower levels of the virus, this test will be able to detect it."However, it can help us find asymptomatic people who may be highly contagious.
According to Esther Mena, head of the Laboratory at the Quirónsalud Madrid University Hospital, since the coronavirus vaccination began, a small delay in the effectiveness and sensitivity of the antigen tests has been observed.In this way, if we are vaccinated but have symptoms compatible with the coronavirus, it is not unusual that when taking an antigen the result is negative, since the signs we have would be the manifestation that the immune system is doing its job and the viral load it is low.
And we must not forget that the fact that "an antigen test is positive is very reliable; but when it is negative it does not mean that you are not infected; that is why we recommend repeating this antigen test two or three days after the first negative," adds Mena.
Experts point out that taking an antigen test before a family gathering or meeting with friends can give us a false sense of security.Because?Because a negative antigen result is not reliable and does not exempt us from infection, since it can become positive in the following hours.However, if we want to do it anyway, let it be as close as possible to the time of the meeting.
Another way to know if the person has been exposed to the coronavirus is the detection of IgG and IgM immunoglobulins.There are five types of immunoglobulins.IgA, IgM, IgG, IgE (with various subtypes) IgE (related to allergies) and IgD.“These immunoglobulins are proteins that recognize, capture and block viruses so that the cells of the immune system (lymphocytes) can recognize and eliminate them,” says Carnevali.
In the case of infections, such as coronavirus, the blood is analyzed for the presence of IgG and IgM antibodies.“The ones that are generated first are IgM.They usually appear between 7 and 10 days after the beginning of the infection.Later, IgG appear, which remain for life in many infections.And let's trust that in the coronavirus too,” says Catalán.And although it seems that this immunity lasts over time, it is still too early to be 100% sure that this is the case since the coronavirus is a recent pathogen.
According to Carnevali, if only IgM is found when serology is performed on the blood to detect the presence of antibodies, it is very likely that the infection is in its initial stages.This immunoglobulin becomes negative over the course of the disease.However, if both IgM and IgG appear, it is possible that “more time has passed since the beginning of the infection and that there are remnants of the acute phase.When only IgG antibodies are observed, we think that the viral replication phase has passed,” says Carnevali.
So what do the positives and negatives mean?Several options can be given:
If IgM and IgG are positive, the person has a relatively recent infection and is developing antibodies.
If IgM is positive and IgG negative it may mean a very recent infection.In this case, it is advisable to repeat the analysis after fifteen days to check if the IgM becomes negative and the IgG positive, which would imply the appearance of antibodies.
If the IgM is negative and the IgG positive it shows that the person has been in contact with the virus and has generated antibodies.“It is the ideal situation and in principle the person has protection against a new infection,” adds Catalán.
If IgM and IgG are negative, the person has not been in contact with the virus and has not developed antibodies.
Catalán warns that IgM can give false positives and lead to confusion and attributes this to the low specificity of the test.However, the outlook may still change since the technology used for such a new virus may advance in the near future.
The same happens with rapid tests, which, although convenient, are less reliable than laboratory tests.This type of test "does not identify the RNA of the virus but rather detects the antibodies produced against the virus in the blood, but it does not quantify them, it only says whether they are present or not," says Carnevali.
Although there has been some confusion in recent months with what rapid tests are, Delgado-Iribarren remembers that these show information about the generation of antibodies and whether the infection has occurred in the previous 7-10-14 days.It is a very simple test that is performed by pricking the patient's finger and putting the blood into a device, very similar to how blood glucose controls are performed in diabetes."Rapid tests are not used to detect active infections. They are very useful for seroprevalence studies, although they are less sensitive than when analyzing the presence of antibodies with a complete blood test."
Thanks to the analysis of IgG, it has been proven that 15% of the population does not generate antibodies.Because?According to Catalán, this may be due to the need for more advanced tests or to the fact that the immune system has used other means of defense, such as cellular immunity;"In the case of the body having to confront the virus again, the immune system will activate other cellular pathways to defend itself against the infection."
A new antibody test, designed by the National Center for Biotechnology (CNB) of the Higher Council for Scientific Research, and Inmmunostep, now allows the identification, quantification and differentiation of antibodies produced by the coronavirus vaccination or by the natural infection produced by Covid -19.
This type of test is based on flow cytometry - a technique present in all hospital centers with clinical diagnostic laboratories - and seeks to obtain very complete information on the immune response to the virus from a blood sample.In this way, and with a reliability of 99%, three types of antibodies (IgG, igA and IgG) and four SARS-CoV-2 proteins (the Spike protein and its RBD domain, both essential as they are the main component) are detected. of current vaccines, and the envelope protein or nucleocapsid (NP) and the protease responsible for virus replication (Mpro/3CLpro).
"After vaccination, antibodies are only produced against the protein used in the vaccine - this is the Spike protein and its RBD domain - while in a person who has suffered the infection, antibodies are also generated against the Mpro and NP proteins," explains José Miguel Rodríguez Frade, CNB-CISC researcher.Obtaining more information about the detection of response to the different virus proteins of which, although knowledge is increasing, there is still much to discover, "will help us improve the understanding of immunity against SARS-CoV-2, something that will be very useful for early classification of patients," says Hugh Reybur, scientist at CNB-CSIC.
The CSIC adds that this device is currently made with patient blood but it is expected that in the future its use in saliva will also be validated.
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