“Test and isolate” It became the mantra that much of the scientific community repeats since the beginning of the pandemic and “swab”, the most used verb since the arrival of the coronavirus. After a year and a half living with Covid-19, almost all of us underwent one of these tests. Yet few today can answer basic questions about the available tests. Between them, what is the difference between a PCR and an antigen test, when each is recommended and if swab sampling is used in both cases. Also: are there other options? What about babies and children?
PCR or rapid test
“There are two types of tests. Those who directly seek the genetic material of the virus and those that track a protein of the Covid ”, he tells Clarion Pablo Bonvehí, Head of Infectious Diseases at CEMIC and member of the Scientific Committee of the Vacunar Foundation.
The first is the famous PCR, which can be done from a nasopharyngeal swab, inserting a swab about 4 centimeters through the nose. “It is also possible to analyze the genetic material from the saliva. 2 cubic millimeters should be placed in a container. This technique requires several steps. That is why takes between 4 and 5 hours”, Details Bonvehí.
Generally, people who take the test in the morning have the result in the late afternoon and those who do it in the afternoon, receive it the next day. “This has to do with the fact that accumulate multiple samples to process all together ”, confirms the specialist. Initially, swabbing included taking the sample by nose and mouth, over time it was determined that it was not necessary.
Antigen test in Costa Salguero. Photo German García Adrasti
As a cheaper and faster alternative, although less sensitive, there is the antigen test or rapid test, which is also done from a swab. “In this case, the objective of the study is to detect the protein S, which is the part of the coronavirus that binds to the cell and leads to infection. The process to obtain the result is much simpler so can be done anywhere and the answer is in 20 minutes”Explains Bonvehí about the method that, for example, is now being used to test passengers arriving from abroad through Ezeiza.
Sensitivity varies depending on the type of test. The presence of symptoms also plays a role. In fact, the antigen test is quite more effective when the swab has symptoms. Part of the explanation is that “having symptoms surely have more viral load, which helps to detect the protein ”, says the CEMIC specialist.
In what situations do you have to be tested?
People should swab when they present symptoms, if they are close contact of an infected person or after a trip. Testing before vaccination is not recommended.
“Is considered close contact to one who was for 15 minutes without the recommended protection to less than two meters away of a infected person, even when that encounter was outdoors. In other words, going to have a coffee with a person who later tells us that they have Covid is an example of this, ”adds Bonvehí.
PCR test through saliva. / Reuters
In that case, the one who was reunited with the infected must be isolated for 10 days. The general recommendation to get the PCR is to wait to day 7 close contact if no symptoms are present.
However, this changes if the person, for work or other reasons, was seen with others before learning that it was close contact. “Faced with this situation, it is advisable to test before the seventh day to alert those who may be involved in the event of a ‘detectable’, although it is necessary to know that a negative result does not imply that the person does not have Covid. It may be incubating the disease. For this reason, the 10 days of compulsory quarantine must be respected ”, he assures.
And he adds: “If the rapid test is negative, but there is a strong suspicion that the person has Covid you have to do a PCR. And if the PCR is negative but the person has symptoms, it is recommended to repeat it after 48 or 72 hours ”.
Tests and their relationship with vaccines and variants
For Bonvehí the vaccination modifies the scenario. “Having a large number of people on at least one dose changes the landscape a bit with regards to infections and testing. Although evidence is lacking, there is surely people circulating with less viral load. On the one hand, this is good because they would be less contagious but, at the same time, it also reduces the chances that those infected will be able to detect it through an antigen test ”, he warns.
Compulsory antigen test in Ezeiza for passengers coming from abroad. Photo Luciano Thieberger.
With respect to the new variants, among them, Manaus and Delta, Bonvehí affirms that “so far, the studies carried out confirm that do not affect the sensitivity of the tests”.
The recommendation of the WHO is that a sufficient number of daily tests be carried out so that the positivity is by below 10%. Until recently, our country was close to 30%. Today the number fluctuates according to the day but, in general, it is between 15% and 20%.
For Bonvehí, in part it has to do with the fact that “cases are decreasing and the number of tests remains stable.” “It would be important that we continue to approach 10% and that we have the possibility of increasing swabs if the number of cases increases again,” he highlights. He insists on this point, bearing in mind that the slogan continues to be “test and isolate”.
Eduardo López, a pediatric infectious disease specialist at Hospital Gutiérrez and a member of the committee that advises the President, says that in recent months the use of rapid tests was included and that this was of great help.
Serological check to detect antibodies in blood. / AFP
“There are places that do not have the necessary technology to do a PCR, so antigen tests are an alternative,” he remarks. And clarifies that do not confuse these tests with serological checks, which are made from a blood draw and are intended to know whether or not the person has antibodies after being vaccinated or having had the disease.
He agrees that the sensitivity is higher with the PCR (of more than 90% considering symptomatic and asymptomatic) although he points out that some recent studies on the use of the antigen test carried out in Spain and the United States also showed good results. “The conclusion was that the sensitivity in asymptomatic cases with the rapid test is between 80% and 85%, when it had been talking about a lower percentage”.
The difference in cost is significant: the PCR comes out close to 7,000 pesos while the rapid test, some 800, according to López.
Regarding the latter, he comments that “less than a month ago, a much shorter swab began to be used in some parts of the world, which is only inserted two centimeters.”
“Not so invasive And it seems just as useful. For now it is only used for the rapid test, “adds the Gutiérrez specialist.
The swab and the boys
López maintains that the boys have to be swabbed. “At one point, this was not done. The directive was to consider them positive if their parents had Covid. This measure was taken not because testing was contraindicated in this group, but because test kits were missing”, He highlights.
At the Gutiérrez Hospital, he says that they are tested for PCR. The sample is generally taken through the nose although, if the child is very bothered, it can also be done through the mouth, as is usually done with the exudate from the jaws to determine the presence of streptococcus ”, he says. And he affirms that it is possible to swab boys and even babies.
In the City, the free swab system is divided into two: there are DetectAR posts and Mobile Testing Units, for asymptomatic patients; and the Febrile Emergency Units (UFU), for those with fever or some other manifestation of the condition. In Province the indication is to approach the nearest post. Prepaid and social works must also cover testing.
The red flags grew over time. Currently, one must watch out for symptoms such as fever (37.5 ° C or higher), sore throat, nasal congestion, headache, cough, shortness of breath, muscle discomfort, diarrhea and vomiting, loss of smell or sense of smell. taste. “It is important do not underestimate mild manifestations and be tested even with a single symptom “, López closes.