Antigen test + PCR = an aberration
Professor Nicolas Vuilleumier, head physician of the Laboratory Medicine Service of the University Hospitals of Geneva (HUG), is categorical: “During the health crisis, the obligation to confirm by PCR any positive antigenic self-test was a waste of diagnostic resources. , contributing to unnecessarily overloading the laboratories. At the height of the pandemic, false positives with these specific tests are very rare. And when the prevalence of the disease is low and people are asymptomatic, it is better to do a PCR to avoid false negatives. Professor Gilbert Greub, director of the Institute of Microbiology at the Vaudois University Hospital Center (CHUV), also points the finger at false negatives: “At the hospital, we were able to observe that two thirds of people without Covid symptoms were infected. by the coronavirus despite a negative self-test on admission, which allowed them to transmit the disease. Make an antigen before going to the cinema: why not. But for a visit to an EMS, it was imperative to do a PCR!”
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The pandemic has put the spotlight on antigenic tests capable of telling, in about twenty minutes, whether one is infected with the coronavirus. These devices have allowed thousands of people to decide whether to forego a family meal or a business meeting. But are these simple and quick self-tests, which do not require the intervention of a caregiver, reliable? With regard to the coronavirus, this point has been controversial. But beyond that, what about these devices sold in pharmacies and on the internet? “The use of self-tests should be limited to the strict minimum, explains Professor Nicolas Vuilleumier, chief physician of the Laboratory Medicine Department of the University Hospitals of Geneva (HUG). Most use low volumes of body fluid, which makes the result less reliable. The patient then finds himself faced with a dilemma: if the result is negative, he cannot completely rely on it. If he is positive, he will have to consult his doctor for a verification. So what’s the point of testing yourself?”
False sense of security
The quality of the sample collected, when the collection takes place and how the test is performed greatly influence the results. “Antigen tests, such as the one used to detect the coronavirus, detect the proteins of the virus or bacteria responsible for the disease. If the quantity of these proteins is not very high, the test may be falsely negative, explains Professor Gilbert Greub, director of the Institute of Microbiology at the Center hospitalier universitaire vaudois (CHUV). It gives a false sense of security.” The specialist takes the example of an outbreak of chlamydiosis cases among young Swedes: “A self-test which detects the presence of the bacterium chlamydia trachomatis is only sensitive in 30% of cases. By using this tool before having unprotected sex, two thirds of infected young people did not know about it and were able to transmit the disease. So you have to be careful.”
There is also the question of the interest of undergoing this type of test for a whole host of diseases or deficiencies. “If the result has no consequence on the attitude of the person taking the test or on their care, why do it? A diabetic has every interest in testing his blood sugar regularly, because he will act according to the result. For the Covid, at the height of the pandemic, a person with typical symptoms had to isolate themselves and take paracetamol. It didn’t make much sense to do an antigen test to confirm what she already knew, ”continues Professor Greub. An opinion supplemented by Professor Vuilleumier: “All tests that cannot be interpreted correctly or that do not change medical care should not be prescribed or performed, including by oneself.”
Without forgetting that it is impossible to follow the evolution of certain endemic diseases if no official laboratory can have access to the results of the tests carried out by individuals.
Jeremy De Mooij, pharmacist in Bulle and delegate of the canton of Friborg at PharmaSuisse, the umbrella of the branch, also shows reserve: “In the pharmacy where I work, we have limited the assortment of self-tests. By doing them alone at home, patients may lack support and run the risk of wrongly reassuring themselves. This is not desirable.”
Interpretation of results and consequences
All these devices are not to be banned for all that. “For example, urine dipsticks can be useful for people regularly prone to cystitis, as long as they have been trained and know the biases of such a test. A positive result allows them to be independent and can facilitate their medical care. On the other hand, what is the use of taking a test to find out if you suffer from a vitamin D deficiency, knowing that more than 80% of the population suffers from it? It is better to take a blood test at the doctor or pharmacy to have an overall view of any deficiencies, to exclude diseases that could be the cause and to supplement accordingly. In general, Jeremy De Mooij understands the interest of promoting accessibility to tests, but remains cautious about a relevant interpretation of the results.
