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Which test is the most reliable? When to do it?

With the Christmas around the corner and after knowing the measurements for contain the spread of Covid-19 during the holidays that the Government and Autonomies have agreed this Wednesday, that they will allow movements between communities to meet with “relatives”, many people are now wondering if a diagnostic test SARS-CoV-2 is a pathway that makes it possible to eat indoors with peace of mind with people who are not living together.

Security measures against Covid-19 proposed by experts for family gatherings.

Making it clear first that Public Health experts recommend this year to restrict contacts as much as possible – “the fewer people, the better, and the fewer households, the better“- and prioritize the encounters outdoor, for those who do not resist sharing a table on the designated dates with their loved ones, taking a Covid test “can greatly reduce the risk of contagion, although it is not an absolute guarantee that it will not occur“, explains Dr. Juan A. Pineda, associate professor of Medicine and specialist in infectious diseases at the Valme University Hospital, who makes it clear that the first option is”spend a Christmas without contact with a family member to risk losing him forever. “

Having said that, If we opt for the test option, which one do we choose? In the private healthcare market, three main types are offered: antigen tests, at a price of approximately 30-40 euros; serological antibody tests, which range from 50 to 100 euros; and the PCR, which costs between 100 and 150 euros approximately.

PCR, the most reliable

The biologist at the Margarita Salas Center for Biological Research of the Center for Scientific Research (CIB-CSIC), Luisa María Botella, who works on a rapid saliva test whose commercialization is scheduled to begin in January 2021, explains that the first option is PCRIt is the “highest sensitivity” test and the one that indicates whether at the time of the test one is infected by the SARS-CoV-2 virus.

PCR has two drawbacks: that it is the most expensive and that it is “a still photo”

“The PCR amplifies, it is like a photocopier of the genetic material of the virus”, says Botella to explain why it is the most reliable and sensitive to detect an active Covid-19 infection. While it has two drawbacks: the first is that it is the most expensive, and the second, that it is “a still photo”. The result can change at any time after the test is carried out if the patient is exposed to a risk situation and it could be the case of arriving infected at a Christmas meeting.

Serological test, to know if the infection has passed

Regarding the serological test, these indicate if the disease has passed and if they already have antibodies IgM (indicating a recent but not necessarily active infection) or IgG (indicating that the infection is over and antibodies have been generated longer), but do not detect whether the patient is contagious at the time of testing.

This test, as indicated by Sanitas, is designed for people who want to know whether or not they have passed the virus and if they have generated immunity.

The combination of PCR and serological test, adds Botella, “would be the most complete, but it depends on the economy of each one.”

Antigen test, the least sensitive in asymptomatic patients

Over the antigen testBottle warns that “the problem is that they are the least sensitive. You have to have enough viral load to test positive, you can be infected but negative for not having enough viral load “, which is what happens with asymptomatic patients.

“The problem is that they are the least sensitive. You have to have enough viral load to test positive”

With her agrees Ester Márquez Algaba, doctor of the Infectious Diseases Service of the Vall d’Hebron Hospital in Barcelona, ​​who recognizes that the antigen test is “fast” and “cheap”, but “only detects positive cases during the first days of symptoms “, so that not reliable enough in asymptomatic people (which will be the case of those who want to meet their family members, since all symptomatic persons must stay at home and contact the health authorities).

How far in advance to take the test?

The CSIC biologist recommends contacting the laboratory in advance to find out how long it takes to have the results. “It would be necessary set the test date to the maximum to have the results as close as possible or even on the day of the trip “, he points out.

Bottle emphasizes that you do not have to travel before knowing the result of the test and advises minimize new contacts from the moment we know the result until we travel to meet up with the ‘relatives’.

27% increase

From Sanitas they assure 20 minutes that “in recent dates the number of tests carried out remains stable —the daily average in the last month is 2,000 antigen tests, 800 CRP and 165 serological tests-, but we are noticing an increase of 27% in the number of requests for information regarding the coverage and use of the different types of tests and in particular about the antigen test “.

From the insurer they emphasize that “the prescription and therefore, the correct use of each type of test must always respond to the clinical needs and suitability of use in each case, assessed by medical personnel skilled”.

Therefore, they continue, “this type of evidence they have to be carried out as long as the criteria are met specified for each of them, doing them on our own and without taking this criterion into account may cause a false trust among citizens“.

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