Covid 4th wave: how and when to use pharmacy tests?

After a significant jump at the beginning of the year, the volume of Covid-19 cases in the country increased again this June.

According to the bulletin Conass (National Council of Secretaries of Health), there are about 35,000 new diagnoses of the disease per day, as indicated by the seven-day moving average (a measure that smoothes the fluctuations of daily records and shows a trend line of pandemic).

The pace is more than twice as high as in April, when Brazil had around 15,000 new diagnoses per day.

Since the outbreak in January and February, the country has had a new diagnostic method — the antigen self-tests, approved by Anvisa this year and sold in pharmacies from March, which can be done at home by the patients themselves.

In what situations are these tests indicated? How do they compare to other antigen tests and PCR? How to correctly interpret the reading cell? BBC News Brasil heard three experts to answer these and other questions about the modalities of diagnosis of the disease.

united by a swab

Both RT-PCR and antigen tests —including self-tests and tests performed in pharmacies and laboratories— are made from a sample taken from the patient’s nose with a long rod, the swab.

But the similarities end there. RT-PCR, also called a molecular test, searches for the RNA of the virus, its genetic material, while antigen tests detect a protein present in the cellular structure of the virus.

In the case of RT-PCR, the sample is processed in a machine capable of multiplying the genetic material collected over a million times. This increases the sensitivity of the test and, consequently, the probability that it will detect the microorganism. There are laboratories that provide the result in up to four hours — during the outbreak at the beginning of the year, however, given the great demand, this deadline was extended to a few days.

With antigen tests, the sample is placed on a piece of paper impregnated with a product that reacts in the presence of a virus protein: a part of the slide (the test “strip”) changes color in a few minutes if the protein be detected.

The experts heard by BBC News Brazil reiterate that RT-PCR remains the “gold standard” of diagnoses.

“For those who can do it, this is ideal”, says Celso Granato, an infectious disease specialist at the Fleury group. “But we know that between the ideal and the viable there is a long distance”, he adds.

Antigen: self-test, in the pharmacy or in the laboratory?

Many people have opted for antigen tests because they are cheaper and have a faster result.

For pulmonologist Gustavo Prado, from Hospital Alemão Oswaldo Cruz, at the current moment of the pandemic, in which the country has ample vaccine coverage – which guarantees that most people who contract Sar-CoV-2 will not have severe forms of the disease. disease—antigen tests, “within limitations,” are a useful tool for detecting mild cases.

Anyone who has a severe manifestation of respiratory syndrome, he emphasizes, whether or not they suspect Covid, should seek a health service.

On the other hand, those who do not have severe symptoms – such as shortness of breath, disorientation and mental confusion, permanent and uncomfortable cough, persistent fever -, he evaluates, can seek antigen tests.

“The big question is to understand that the tests are not all the same and have different diagnostic performances”, he adds. “And it is necessary to go back to the moment of collection, not only to the reagent itself — how sensitive and specific it is. These pre-analytical conditions are fundamental for the result to be credible.”

In this sense, for antigen tests to have a greater chance of success, says the pulmonologist, they need to be performed at the right time (read more below), with an appropriate collection and reagent mixing technique, adequate application in the reading cell and with due processing time.

The director of the clinical analysis committee of the Brazilian Association of Diagnostic Medicine (Abramed), Alex Galoro, also reiterates the importance of quality in performing the different stages of the test, especially when choosing between the self-test, the antigen test carried out at the pharmacy and the antigen test done in the laboratory.

“The laboratory professional is trained, he lives on it, the pharmacy professional may have received some training, but certainly less — so there may be some failure in the execution. In the case of the self-test, even more”, he adds.

The self-test kits for antigen detection, by the way, are different from those used in pharmacies and laboratories, despite using the same mechanism.

For safety reasons, the collection of material in the self-test is not done in the deepest region of the nose, but only in the most superficial area – in general, the instructions indicate inserting the swab between two and three centimeters in the nose.

In practice, this means that the probability that the sample collected in the self-test has a small viral load —which is not even detected by the reagent—is greater than in the test carried out in the pharmacy or laboratory.

“There can be a lot of false negatives”, points out Galoro.

When to do?

In the case of antigen tests, the recommendation is that it be done on the second or third day of symptoms, when there is a greater viral load in the respiratory system of the infected individual and, thus, the probability that it will be detected is greater. Officially, Anvisa suggests that they be done between the first and seventh day of symptoms.

For asymptomatic people, the ideal is to do it between three and five days after contact with someone who has been diagnosed with the disease, due to the incubation time of the virus – which has decreased considerably since the beginning of the pandemic, especially after the emergence of the omicron variant. , responsible for the most recent explosions of cases around the world.

“In the beginning, the incubation time could range from two to even 14 days — this is no longer true. Fleury.

How to read the self test

For those who opt for the self-test, it is important to know how to read the result.

The test slide has two fields: C for “control”, and T for “test”. When the result is negative, the colored line appears only in area C. When positive, both areas are “painted” on the slide.

Anvisa highlights that the result is considered positive regardless of the color intensity of the lines.

The exam is considered invalid if only the T line is colored, but not the control line; if neither is painted or if there is a large smudge on the blade, as shown in the illustration below:

It was positive, now what?

Pulmonologist Gustavo Prado points out that antigen tests are very specific and, therefore, have a low incidence of false positives.

“Positive test is positive result.”

The recommendation is isolation for a period between 7 and 10 days, counting from the first day of symptoms.

Anvisa even issued a suggestion releasing the end of isolation from the fifth day, with a new test that brings a negative result, but the pulmonologist does not consider the measure safe.

“Something between 10% and 20% of patients may still be transmitting the virus on the fifth day. The ideal is that the person completes at least the seven days of isolation.”

It was negative, I don’t have Covid?

Negative results, meanwhile, need to be looked at more carefully, experts say.

The sensitivity of self-tests varies, but many reach around 80%, says Prado — which means that out of every 100 positive cases, the test will detect 80.

“Negative test can be false negative”, he ponders. Depending on the patient’s condition, if it is a case with high clinical suspicion of Covid, it is possible that the health professional will ask to repeat the test or perform an exam with greater sensitivity, such as RT-PCR.

For those who took the test because they are symptom-free and had contact with someone who had a positive diagnosis, the doctor’s recommendation is to maintain care to avoid transmission – use of a mask, hand hygiene, social distance, preference for open environments and ventilated.

At the beginning of the pandemic, the orientation was that those who had contact with someone infected also isolate themselves – a recommendation that changed with the evolution of the disease and with the expansion of vaccination.

Wave of underreporting

The three experts interviewed by the report highlighted that the spread of antigen tests, especially self-tests, has generated a significant increase in the underreporting of new cases of Covid-19, especially of mild cases.

“What worries me about the self-test is that not everyone notifies the health authorities when the result is positive”, says infectious disease specialist Celso Granato. “Here at Fleury, we are seeing a positivity of 50% in the tests, a very high percentage, which indicates that the disease is widely disseminated among the population”, he adds.

“This peak [de casos] is undersized”, agrees Galoro.

Anvisa does not consider self-tests as diagnostics for Covid-19. The recommendation is that, once the test is positive, the patient seeks a confirmation diagnosis in a health service. In real life, however, many of the people who test positive for self-tests, especially in mild cases of the disease, remain in isolation at home.

The need for “confirmation” of the result by the health system, in Prado’s view, is “a redundancy that burdens the system” and can saturate it. In parallel, he adds, the positive test that is not counted “is a diagnosis that is not necessarily generating knowledge [em relação à situação da pandemia no país] or change of plan”.

‘This text was originally published in https://www.bbc.com/portuguese/geral-61913887‘