The first thing you must understand is that a PCR detects the genetic material of the coronavirus, its RNA, and can measure the amount of virus that is in an organism, that is, the viral load. What does it mean for a PCR to be negative? It means that, at the time the sample was taken, the test did not detect the virus in our body. And that can be the case for two reasons, because you do not have the virus or because the viral load is so small that it is undetectable. That is, that negative does not mean that after the sample is taken from us we cannot be infected. This could even happen immediately after taking the sample. If what has happened for the test to be negative is that there is a certain viral load, but it is so low that the CRP does not detect it, there may be several reasons. It may happen that the person from whom the sample has been taken is very early in the infection and after a few days the viral load may rise and be already detectable. It can also be undetectable because the immune system has already attacked the coronavirus and defeated it.
In the case of the antigen test , it is also used to detect if we are infected. What this test is looking for are the proteins that coat the genetic material of the virus, which are like the carcass of the virus. As with PCR and RNA, if you have these proteins in your body it is because you have the coronavirus. In this case it is exactly the same as in the PCR. A negative means that at the time of collection of the sample you are not infected or that the viral load is so low that the test cannot detect it. And this is also more likely to happen with this test because it is less sensitive than PCR. In other words, PCR is capable of detecting smaller amounts of virus than the antigen test. But a negative in the antigen test is due to the same reasons as a negative in a PCR: either you do not have the virus or the viral load is so low that the test could not detect it well because you are very early in the infection, well Because your immune system has managed to overcome it
In both cases, if the result is positive, the person who has been tested has the virus
The antibody test is a totally different test because what it shows it is our immune response against the coronavirus. And here are two options. The test can detect IgM, which are fast-response immunoglobulins and, as their name indicates, is the protection that our body sets in motion when it detects the virus. When the test is positive for IgM it means that the infection is ongoing. But the antibody test can also detect IgG, which are slow-responding immunoglobulins, which are generated when the infection is already ending or has ended and are those that produce long-term immunity. A positive for IgM means that the virus is most likely in your body. A negative in IgM means that at the time of collection of the test you did not have the infection, that is, you did not have the coronavirus. And a negative in IgG means that you either have not had the infection or it is so recent that the defense mechanism with slow responding immunoglobulins has not yet been activated. That is, the antibody test measures your immune response against the virus and at what point in the infection you are, whether it is ongoing or you have already passed it.
Elena Senís Herrero is a researcher at the Vall d'Hebron Institute of Oncology
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