Antigen test: When you have to repeat them if they are negative

No one talked about antigens, and, of course, becoming a test was not an indispensable requirement to go to a party or a meal.Now, however, these self -diagnosis tests have become our inseparable friends and even have a fixed and maximum price set by the Government in 2.94 euros.

Desde que la última variante del coronavirus, la ya famosa ómicron, multiplicara los contagios hasta cifras nunca vistas desde que comenzara la pandemia, ya es habitual el autodiagnóstico en casa con los test de antígenos que se compran en la farmacia. Sin embargo, ¿Cuándo hay que realizar el test? Y, sobre todo, ¿cómo hay que hacerlo? Más importante aún, ¿qué significan los resultados? ¿Si estás vacunado tardas más en dar positivo? Esther Mena, jefe del Laboratorio del Hospital Universitario Quirónsalud Madrid, y Marta García Coca, microbióloga del Hospital Universitario Quirónsalud Madrid, resuelven a CuídatePlus estas dudas.

Are the antigen tests reliable?

Antigen tests diagnose active infection, by detecting the presence of virus proteins in the mucous membranes in a few minutes.So they are reliable.But, eye, not all results are.Keep in mind that its predictive value is very high if the result they throw is positive.However, the fact that when the test is done, the result is negative does not imply that we have not infected with the COVID-19. Por eso tenemos que tener en cuenta que esta prueba sólo es válida cuando da positivo, al contrario que la PCR cuyo valor predictivo cuando es negativa es muy alto.

Hence it is recommended to repeat the antigens test two or three days after the first negative, says Mena.

When do you have to perform an antigen test?

Según la Agencia Española de Medicamentos y Productos Sanitarios el test de antígenos debe realizarse en los siete días posteriores a la infección o en los cinco primeros días con síntomas.

No obstante, según apunta Mena, antes de la vacunación frente al coronavirus, los test de antígenos realizados desde el primer día de aparición de los síntomas y hasta el quinto tenían una gran eficacia y sensibilidad.However, "now with vaccination we are observing that those times are being delayed a bit".Hence, even having symptoms - general rogue, cough, throat itching, headache, and even gastrointestinal symptoms that were not common before - the antigen test shows a negative result.

Why does the test negative if there are symptoms?The chief of the Quirónsalud Madrid laboratory explains that “the symptoms are produced in response to the immune system that the virus detects before and are not only mediated by the viral load.Being vaccinated, when the virus comes into contact with the organism activates the immune system and we start with symptoms much earlier;However, at first the viral load is low (being immunized) so the antigens test can give us negative despite being infected ”.

If we do them before a dinner or social event, we must not forget that the fact that they give negative does not imply that we cannot positivize tomorrow and be infected.In addition to being contagious.

Do antigens test detect high and low viral loads?

Antigen tests are able to diagnose, to date, all the variants that have appeared from the SARS-COV2, including joy, when detecting the basic form of the virus and its proteins.However, these diagnostic tests "only detect very high viral loads," adds García Coca, who emphasizes that that is why a positive test is reliable but a negative does not rule out the infection.

How to do well and how to read an antigen test?

Once we have the test at home and we are going to perform the test the first thing we must do is read the instructions that accompany it either by sample of saliva or nasopharyngeal.The tests acquired in pharmacies, the only authorized place for sale in Spain, must include the CE marking and its identification digits.

The self -diagnostic kit is accompanied by a test tube, a solution (liquid), a coast and the cartridge or bath bathtub.It is important to follow the instructions that accompany the device to prepare the solution in which we will introduce the stick or swab.It is worth having a clock to measure the performance time indicated by the antigens test.From the AEMPS they advise to perform properly, always according to the instructions, the sampling."A poor quality sample can lead to a wrong result.To prevent them from degrading, do the test immediately after taking it ”.

Once we have everything prepared, you have to collect the sample with the swab.According to Mena, “Antigen tests should not be performed in the nasal pit, but should reach the nasopharynx.The sample thorunda must be taken to the rear of the nose and enter the nasal interior ducts.It is recommended to rotate the tab to obtain the sample of the tissue and keep it a few seconds.To ensure that the sample is collected correctly we can also do it in the two nostrils ”.

The swab or baston must be introduced into the solution or buffer.Mena remembers that you have to keep the time established the instructions and pour it into the bathtub or cassette.

What do C and T mean?

From there you have to observe the two lines of which the test.The manufacturer will indicate how much time you have to wait for the results to be reliable.It is important to keep in mind that reading should not be done later of the established time.The control part is indicated by a C and tells us if the test is well done.The part of the T is that of the test."If a line does not come out in the test line is that it is negative, if the line comes out in the test line, it is positive," says Mena.

Why are the lines not marked?

If the control line is not marked in the results window, it is a clear proof that the test has not been done correctly.It may also be due, Mena adds, to the bathing or buffer being defective, but “in most cases it is because there is not enough biological material to perform the test.You have to repeat the test from the beginning with another test ".

In the event that in the test window (t) the brand or little marked is seen slightly, this may be due, García Coca adds, to “that there is little viral load.It is very likely that if this test is repeated the next day the most marked (positive) line appears or the line disappears (negative).It depends on at what time the infection is done the test, if it is very at the beginning or at the end ”.This means that how much more marked is there more viral load?In this regard García Coca states: "It is not totally accurate, but it is usually like that".

However, you have to look good to detect if that line appears, even very lightly, since that indicates that the virus is present.

Usual precautions

Do not ignore the measures that have proven effective to date: use of mask, interior ventilation and maintain distance with others. Y tampoco hay que olvidar que si vamos a realizarle a otra persona un test de antígenos es conveniente cubrirse con una mascarilla FFP2.