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Date Published: 20/12/2021

How to correctly use a Covid antigen test

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Covid tests are available over the counter in pharmacies across Spain

How to correctly use a Covid antigen test

The lead-up to Christmas and the emergence of the omicron coronavirus variant, which the experts believe is far more transmissible than the original strain, have resulted in a surge in demand for over-the-counter antigen tests, which can be purchased in pharmacies across Spain for as little as four euros.

However, the Spanish Agency for Medicines and Health Products (Aemps) has warned that these self-tests can often result in a false negative, particularly in asymptomatic patients or those with low viral loads, and so has issued a series of guidelines to ensure that the tests are carried out correctly.

Generally speaking, antigen tests should be carried out in the first seven days after infection or in the five days after the appearance of symptoms for best results.

What the kit contains

Depending on the manufacturer, each kit usually includes an antigen test cassette, which will come in a sealed bag, a sterile swab, an extraction tube, and an extraction solution. Sometimes, the box itself serves as a tube holder.

Steps to follow

It is essential that the procedure is followed to a tee to ensure the test results are as accurate as possible. To this end, many manufacturers recommend having a stopwatch to hand to time the process. In addition, hands should always be washed before beginning the test and the patient should blow their nose several times.

If the kit contains extraction solution separate from the tube, it must be opened carefully so as not to spill any liquid. The entire contents must then be poured into the tube.
Open up the sealed container of the sterile swab and locate the tip without touching it.
Gently insert the swab into one of the nostrils, at least 2.5 cm deep. Rotate the swab 3-4 times to collect as much sample as possible. Repeat the same procedure for the other nostril.
The next step is to insert the swab into the extraction tube, rotate it three to five times and leave for one minute.
Remove the swab and discard it. The tube should contain a dropper cap, which will be used to deposit the solution on the cassette.
Open the container where the test cassette is located and place three drops of the sample in the well indicated for it (in some tests it is indicated with the letter S), lightly squeezing the extraction tube.
The result will be displayed after about 15-20 minutes.

Interpreting the results

All tests cassettes contain a white strip with two letters: C and T. The letter C is called the control zone, and the line that will appear marked next to this letter indicates that the test has been carried out correctly. These are the possible results that an antigen test can give:

Positive: If two lines appear after 15 or 20 minutes, one in the control zone (C) and the other in the test zone (T). In this case, the test is considered valid and positive.
Negative: If after 20 minutes a line appears in the control zone (C), but no other is observed on the T, it means that the test is valid and negative.
Invalid: If after the same period of time no line appears in the control zone (C), the test result is considered invalid and the test must be completely repeated.

What to do in the case of a positive result

Antigen tests are not considered a diagnostic test, and if the result is positive, the experts recommend booking a follow-up PCR as soon as possible.

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