Taking an antigen test before having dinner as a family on Christmas Eve: these are the deadlines and situations that you have to know in order for it to make sense

At the beginning of December, a Christmas meal among health workers at a Malaga hospital ended up causing an outbreak of more than 70 infected. According to these health workers, they went to the food after taking an antigen test and giving all negative results. However, a few days the confirmed positives began to occur.

This example can illustrate how taking an antigen test may not be a guarantee, especially if we do it at the wrong time, when it can be counterproductive, since it is possible that we do it in advance giving a negative result … but it becomes positive a few days later, when we attend a dinner with other non-cohabitants, even if we self-define the previous days.

Counting the days for the antigen test

The main risk is in the first days after infection. At that time we are already carriers of the virus, but no symptoms have appeared and the viral load is reduced, to a level that neither a CRP nor an antigen test can detect positivity. That is why it is possible that even if we do a pharmacy-bought antigen test and the result is negative, everything. But they can still be infected, and infect others.

WHEN would I take an antigen (self) test?
(* The values ​​are approximate and take into account what I am reading about the delta variant.)

🤯1) If I had symptoms, even mild ones.
(Although it is not easy to distinguish covid symptoms from other common disease symptoms)

– Maria I. Tapia (@mariaitapia) July 30, 2021

In the case of close contacts with positives or suspicions of having been infected (for having been in a risk context, such as with a crowd without a mask for hours in a closed space), an antigen test could not tell us with any reliability if we are infected or not until the third or fourth days after contact, and up to ten days later. In the case of not having had close contacts or being exposed to risk situations, if we experience symptoms compatible with COVID, we would not have to wait, since in that case an antigen test could detect the virus.

One possible situation for Christmas is to take an antigen test to be able to dine with the family on Christmas Eve with the lowest possible risk of contagion. This makes sense if we do the test as soon as possible, almost before the family reunion, and if we are not going to live with them in the following days.. In that case, we may reach Christmas Eve in the incubation phase, without the ability to infect them, but a few days later we do have it, even though we are asymptomatic.

The results of these antigen tests, or of PCR tests in the case of attending a health center indicate:

• Positive PCR: carrier of the virus, most likely contagious.
• Negative PCR: we are not in the contagion phase, but perhaps we are incubating it.
• Positive antigens: carrier of the virus and contagious.
• Negative antigens: not contagious, although possible carrier of the virus.

And here is the table of our colleague Javier Jiménez with the details of each of the tests, including the antibody tests, which do not serve to detect if we are infected.