Home Covid-19 Covid-19 tests: a guide not to get lost

Covid-19 tests: a guide not to get lost

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As with all infectious diseases, the key to containing Covid-19 is to detect cases of infected people, isolate them and trace all their contacts in order to break the chain of transmission. The earlier and faster the detection, the greater the effectiveness in preventing the virus from spreading.

Since the beginning of the pandemic, the quintessential test used to diagnose the disease has been PCR. However, there are other screening options: from the use of antigen tests – the so-called rapid tests – to PCR with saliva samples.

To this is added that, in EE.UU., the emergency use of a diagnostic self-test based on a technology similar to PCR was approved this past week. And the experts consider that it would be expected that the European Medicines Agency (EMA) will end up approving their use in the coming months and that, therefore, they will begin to be distributed by Europe. But what is each of the tests for? Are the rapid tests that can be purchased online reliable? Is a PCR in saliva the same as nasopharyngeal?

Two types of tests are used to diagnose whether a person is infected with SARS-CoV-2: those that detect genetic material of SARS-CoV-2 or those that identify the proteins of this virus, the so-called antigens. Regarding the former, although there are several, PCR is the most used. It is based on repeatedly copying (amplifying) the viral RNA until it reaches detectable levels. It is the most sensitive and specific: it manages to detect more than 90% -95% of infections. In return, it requires between 4 and 6 hours of processing the samples in a laboratory, and the population usually receives results between 24 and 72 hours after being tested. It costs between 30 and 70 euros.


It is the most reliable test, which also allows finding asymptomatic patients

“It is the most reliable. It allows detecting individuals with symptoms and also asymptomatic ones, which is why until now it is the one used to carry out massive population screening ”, he explains Juanjo Garcia, head of pediatrics at the hospital Saint John of God.

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The samples used are from the nasopharyngeal mucosa, although saliva will probably begin to be used; Sant Joan de Déu carries out a study to validate whether PCR in saliva samples are as sensitive as using nasopharyngeal tubes. “[Las de saliva] they are less invasive and annoying, you do not need trained personnel or so many protection measures because it reduces the possibility of contagion by not generating aerosols – García details. It will become the standard. “

Another way to diagnose Covid-19 is antigen testing. They detect around 70% of cases in the general population and are indicated for people with clear symptoms of the disease. To be effective, it must be done between 5 and 7 days after the first signs of infection have appeared and also require taking a nasal or nasopharyngeal swab.

“They serve to detect people with a high viral load, who are the most infectious,” he points out. Adelaida sarukhan, PhD in Immunology and Scientific Writer at the Instituto de Global Health (ISGlobal) from Barcelona. “They are less accurate than PCR, but faster: in 30 minutes you have the results and that allows the person to be confined at the moment and thus avoid infecting more people,” he adds.

Antigen tests

With a sensitivity of 70%, they are indicated for people with symptoms

The main disadvantage of these rapid tests, with a cost of around 4 to 6 euros, is that it is not known how reliable they are in identifying asymptomatic people, who are the great unknown in this epidemic.

“In some tests that are purchased online that are not validated, their sensitivity could drop to 50%, which is equivalent to tossing a coin to decide whether or not you are infected,” says Jordi Vila, head of the clinical microbiology service of the hospital Barcelona Clinic and director of the viral and bacterial infections program at ISGlobal, which recommends, in view of Christmas, not to trust the results of these tests when deciding whether to attend a family meal.

In this sense, tests such as the rapid diagnostic kit approved this week by the Food and Drug Agency americana (FDA), with a very high sensitivity and results in 30 minutes, could be a useful tool to decongest health services – it is done at home – and help stop the epidemic. This self-test is based on a technique similar to PCR, called LAMP: the person, by medical prescription, takes an oral sample (rubs the walls of the mouth with the swab for a few seconds to drag cells from the oral mucosa) and the inserted into a cartridge that detects the presence of the virus in less than 30 minutes.

Diagnostic self-tests

Rapid tests based on LAMP and CRISPR could be approved

“This type of test will arrive here safe and even much more sensitive than the one approved by the FDA”, considers Vila, who explains that Dutch researchers have worked on a LAMP-based diagnostic self-test kit incorporating the CRISPR gene editing technique. – winner of the Nobel Prize in chemistry this year – to increase detection sensitivity. “The results are very positive, the sensitivity is very high and in 40 minutes it yields results”, he adds and points out that for this type of test to be used in a massive way, it must first lower the price: the FDA test is at 42 euros and for Vila they should not exceed 10 or 15.

In any case, whatever technique is used is used, “it is crucial that traceability be ensured, both for transmission control and from an epidemiological point of view, to know in real time how the epidemic is behaving”, insists Vila .

Joint PCRs are a good option to lower costs and be able to carry out frequent massive screening: taking samples from groups of people, such as students in a class, salivate each one and analyze them together. If negative, 20 individual PRCs have been saved.

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