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“Test, test, test.” This is how Tedros Adhanom , director general of the World Health Organization (WHO), urged the countries  at the beginning of the covid-19 pandemic. Testing, isolation and contact tracing should be – and have been in many cases – the “backbone of the global response” to the crisis, according to the head of the agency.

According to experts, there is no global consensus on the strategies of these tests, combined with robust public health measures

Today, despite the fact that there are more than  1,000 brands of diagnostic tests  available on the market – and others in the pipeline – many countries face common challenges: the ability to perform tests, global competition for the access to diagnostic kits and supplies (including swabs for sample collection), choosing the right test for each situation, and ensuring that tests are externally validated.

According to experts, there is no global consensus on the strategies for these tests, combined with robust public health measures, such as quarantine and contact tracing. However, testing outside of healthcare settings has been done on an unprecedented scale.

Faced with this situation, an international team of scientists has published a review in  The Lancet  on the strengths, weaknesses and applications of the different covid-19 tests, as well as their potential for managing the pandemic since Diagnostics have proven to be key in the  response to the pandemic .

Covid test

Three tests for different contexts

There are three main methods for the detection of SARS-CoV-2 infection and their role has evolved during the course of the pandemic. According to the Lancet study  , these three types of COVID-19 tests continue to play a key role today in the transition from pandemic response to pandemic control.

The  molecular tests such as  PCR , are highly sensitive and specific for detecting  viral RNA , and WHO recommended to confirm the diagnosis in symptomatic individuals and to enable public health measures.

Lorem ipsum pain sit amet consectetur et adipiscing elit lorem ipsum pain sit amet conse turum unionBLANCA LUMBRERAS, Professor of Preventive Medicine

The  antigen tests  detect  viral proteins  and although they are less sensitive than molecular tests have the advantage of being  easier  to perform, give a result faster. They also have a lower cost and can detect infection in people with a higher risk of transmitting the virus to others.

Antigen tests “are useful when we have compatible symptoms,” Blanca Lumbreras , professor of Preventive Medicine and Public Health, explains to SINC  , independent of this research. Also, in the event that Primary Care is saturated and it is not possible to have a result as soon as possible, “if we have been in close contact with a positive person”. 

Therefore, they can be used as a public health tool for the  screening  of people at higher risk of infection, protect clinically vulnerable people, guarantee travel safety – although they do not allow obtaining the COVID passport – and the resumption of the school and social activities, and allow economic recovery, point out the authors of the work.

However, these tests “are not useful as screening tests in  asymptomatic patients  or  without close  close contact in time,” recalls Lumbreras, “since they can give false negative results” in which the test tells you that you do not have the infection when you do.

With the deployment of the vaccine,  antibody or serological tests  (which detect the host’s response to infection or vaccination) can be   useful surveillance tools to inform public policy. However, they should not be used as proof of immunity, as the correlates of protection remain unclear.

How to interpret the results

In the case of antigen tests, “it is important to know that if the result is  positive  the probability of being infected is very high, and you should contact Primary Care, in addition to following the isolation instructions”, indicates the professor. 

If the result is negative, infection cannot be ruled out due to the probability of these tests giving false negatives.

If the result is  negative , “infection cannot be ruled out due to the probability that these tests give false negatives.” Therefore, “if you have symptoms or have had close contact, you should contact Primary Care,” he advises.

In addition, the sensitivity  and  specificity  of these tests must be taken into account  . Sensitivity is the ability of the test to give a positive result when there is an infection: if it is low, there will be false negatives. Specificity is its ability to give a negative result when there is no infection: if it is low, there will be false positives.

 When it comes to results, they are always probabilities,” Lumbreras concludes. Explain that the percentages are average values ​​obtained from previous studies, and that they can vary between different jobs.

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