[ad_1]
What kinds of tests exist and what are they for?
We can divide the tests for the new coronavirus into two broad categories: tests that aim to detect the presence of the virus in our body and those that detect antibodies (IgM, IgG and / or IgA) produced by the body in response to SARS infection- CoV-2, the so-called serological tests. Tests that detect an active infection look for components of the virus and are divided into two classes. There are molecular tests that look for the genetic material of the virus itself – in this case, ribonucleic acid (RNA) – that uses a technique called “polymerase chain reaction” or RT-PCR, as they are known. And antigen tests that detect proteins on the surface of the virus.
What kind of samples are needed to perform the different tests?
Molecular and antigen testing is usually performed on samples from the upper respiratory tract, collected with a swab (a kind of long swab) that is inserted through the nose or mouth into the pharynx. Serologists analyze blood components.
What rapid tests are there?
One of the confusions that exists between the different types of tests has to do with the “rapid test” designation that can be applied to both antigen and serological tests. In both cases the diagnosis is made in vitro, usually individually, and is similar to a pregnancy test. Results take between 10 and 30 minutes. RT-PCR is always run on automated laboratory equipment. Antigen tests are also performed in the laboratory, which indicate the amount of antibodies found and not just whether or not a type of antibody was detected. But these are less widely used than rapid tests, which tend to be less sensitive than antigen labs. On the other hand, there are systems based on the RT-PCR technique that allow results to be obtained in a shorter period (less than one hour), but they remain “automated” and do not fall into the category of “rapid test”.
What are the most reliable tests to detect a new coronavirus infection?
RT-PCRs are still considered the gold standard tests, as they are more reliable. Antigenic antibodies are, in principle, less sensitive than molecular tests (RT-PCR), especially when the viral load is lower. Thus, early in infection, detection of the presence of a SARS-CoV-2 protein may occur later than identification of RNA by PCR. The stage in which it is most likely possible to detect a viral antigen in an infected individual, using a rapid antigen test, is when the viral load is typically highest, that is, in the first five days after the start of the symptoms of the disease.
The General Directorate of Health prefers to carry out molecular tests (for example, in cases of patients with hospitalization criteria or in asymptomatic patients with high-risk contacts with confirmed cases) and only if RT-PCR is not available or not can respond in less 12 or 24 hours, depending on the situation, the use of rapid antigen tests is recommended.
But no test is 100% reliable. Even RT-PCR may not detect infection on the first day or two after contamination, when the virus is still beginning to replicate in the body. The quality of swab collection is also critical to achieving a reliable result, both for molecular and antigen testing.
If they are less reliable, why do authorities want to use antigen tests?
The first rapid antigen tests to appear produced many false negatives, leading the scientific community to look at them with suspicion. But as the technology evolved, new tests of this type appeared, with better levels of sensitivity.
There are manufacturers that even guarantee that their test detects 97 out of 100 positive cases, but in reality, independent studies have shown a lower sensitivity rate, than according to a specialist from the National Institute of Health Dr. Ricardo Jorge, if between 80 to 90% . The difference is essentially related to the type of people taking the test: if all are symptomatic, the test is more effective than including asymptomatic people. But there are advantages: these tests are much cheaper than RT-PCR and give a result in no more than half an hour. Speed can be an important advantage when taking public health measures such as detecting an outbreak in a home or school and separating patients with higher viral loads early.
How much do the different tests cost?
Antigen testing is much cheaper than RT-PCR. The Red Cross carries out antigen tests for 20 euros in different parts of the country, a third of the amount it charges for RT-PCR (60 or 65 euros). Sixty-five euros is the amount that the State pays to private laboratories for RT-PCR tests carried out at the request of the National Health Service. For individuals, the largest network of laboratories charges 100 and 101 euros. On the other hand, antigen test results take only 10 to 30 minutes, which can be important for quick public health action. Serologicals are sold in some pharmacies and clinics and usually cost between 20 and 25 euros.
Why shouldn’t serological tests be used to diagnose novel cone virus infection?
These tests detect the presence of IgM, IgG and / or IgA antibodies against the new coronavirus, that is, the response that the body gave to the SARS-CoV-2 infection. The immune response in SARS-CoV-2 infection is detected from the second week of infection, that is, generally between eight and ten days after the onset of symptoms. The problem is that before this period of time a negative test does not guarantee that the person is not infected. Only, at that point, he has no detectable antibodies. The person may already be infected and do not yet have antibodies to the disease. The negative test can give a false sense of security, which can lead people to decrease basic care (maintain a safe distance, wear a mask and wash their hands frequently) and contaminate those close to them. There are tests that allow you to see the type of antibodies detected, IgM and IgA are usually associated with an active infection and IgG with a past infection. But as already mentioned, the person may not yet have IgM and IgA antibodies and may already be infected.
What are serological tests for?
At the individual level its usefulness is very limited. But its use may be justified in the context of population epidemiological studies to understand what percentage of the population has been infected with the virus (since it is known that cases confirmed by tests always show only part of reality) or in research who want to monitor the immunity against SARS-CoV-2 in the population.
