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Antibody tests sold in pharmacies: what they are for and how to interpret the results

The emergence of Covid has also brought with it a series of terms that, in many cases, can lead to confusion or, at least, raise doubts. Until now, certain tests such as antigen tests or serological tests were limited to the strictly healthcare field and, therefore, it was enough to place trust in professionals. But now, both can be purchased in pharmacies to carry out the diagnosis at home and, if in the case of antigen tests their usefulness is more or less clear: knowing whether or not one is infected, serological tests can generate certain questions or, on the contrary, raise expectations that later come to nothing.

The so-called rapid antibody tests reveal whether the person has developed antibodies against Covid. But, here comes the first point: they do not detect the presence of the coronavirus, but rather they reveal the individual’s immune response to the virus, as specified by the Spanish Agency for Medicines and Health Products. That is, they do not serve to determine whether at the time the test is performed you are infected or, in other words, there is an active infection, but rather they determine whether the body has generated different types of specific antibodies against the virus. and, if so, it indicates that there has been an infection with the virus even if no symptoms have occurred.

However, there is another factor to take into account and that the Medicines Agency highlights. Since the protective effect of antibodies against SARS-CoV-2 over time continues to be investigated by the international scientific community, “the detection of antibodies does not indicate that protective immunity has been achieved.”

On the other hand, the reliability of this type of test, which must be purchased only in pharmacies, depends on its sensitivity levels, which must be specified in the instructions for use. The higher they are, the safer they will be.

What the results indicate

Serological or antibody tests detect two types of antibodies: IgM (immunoglobulins that appear in the early phase of infection and disappear after a few weeks) and IgG (immunoglobulins that appear in the late phase of infection and remain for a long period of time, sometimes even years.

Blood samples are generally used for rapid antibody tests. You must follow the instructions for use of each product although, in general, they are obtained with a small puncture with a lancet.

What does a positive result mean?

When it offers a positive result for IgM antibodies, it indicates that the person has been exposed to the SARS-CoV-2 virus recently, although that does not mean that the disease is ongoing. In fact, as indicated by the Spanish Medicines Agency, during the first days of infection it is undetectable, so a negative result does not exclude the possibility of being infected. In fact, the presence of IgM can coexist with an ongoing infection and remains detectable several days later once the disease has been overcome.

If the result is positive for IgG, it means that the person has been exposed to the SARS-CoV-2 virus, although not necessarily recently since, in fact, this antibody is usually present during the final phase of the disease and after a long period. of time later. For this reason, and as with the previous indicator, the contagion is not necessarily active but rather the infection could have occurred months ago.

There is even a time, during the infection or once it is over, when the results for both IgM and IgG are positive simultaneously.

Be that as it may, a positive result cannot lead to a false sense of security when thinking that the antibodies will free people from a new infection since, as experts point out, “the fact of presenting antibodies does not indicate that a cure has been achieved.” protective immunity.

What does a negative result mean?

In this case, for both IgM and IgG antibodies it implies that no antibodies have been found against Covid, so, in principle, one has not been exposed to the virus.

Can a false positive occur? And a false negative?

Yes. The test may give a positive result in people who do not have antibodies against Covid-19. The reason: past or current infections caused by other coronaviruses or other interfering substances.

It can also give a false negative in people who have generated antibodies against the coronavirus. Mainly, it is due to the presence of substances in the sample that inhibit the reaction.

Are they used to ensure that the vaccine has worked?

A priori, no. The Covid-19 vaccines currently marketed generate antibodies against a very specific antigen of the virus: the S antigen, known as the spike. Therefore, only serological tests designed to specifically detect IgG against the S antigen will be useful to know if the injections received have fulfilled their purpose.

If this is the objective pursued, it is important to read the instructions for use of the product and check that they detect anti-S IgG and that they are intended by the manufacturer to detect the immune response to vaccination.

These are new generation antibody tests, while IgG/IgM tests do not specify which viral antigen the antibodies are against and do not serve to evaluate the immunity generated by the vaccine and would only serve to detect exposure to the virus.