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The arrival of Omicron to our lives, a little over a month ago, has led to a revolution in the management of the pandemic of Covid-19. From changes in symptoms and their severity (now generally milder) to the duration of quarantines, which have been reduced. And that despite the fact that there are still more uncertainties than certainties about this new variant, as the vice president of the Spanish Epidemiology Society, Óscar Zurriaga, warned on Tuesday in this other article on the convenience of shortening or not the isolation of positives . The way of reporting positives has also changed. It is no longer necessary to confirm with laboratory test.
The results of the self-tests, in situations in which the autonomous communities cannot guarantee confirmation in time by PDIA, and that, therefore, are handled as Covid cases, will be considered confirmed.
Now, with a virus spreading faster because it affects the upper respiratory tract more than the lower respiratory tract, the question has arisen as to whether the respiratory tract will be more effective. saliva test than traditional nasopharyngeals to detect a positive.
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small study, which has not yet been peer-reviewed, suggests that most Ómicron cases can be infective days before being detected by a rapid antigen test. The investigation was done with 30 people who were infected in five companies where there were outbreaks last month. A PCR was made from a saliva sample and a rapid antigen test was performed by extracting a nasal sample. The PCR test found positives three days earlier than antigens. In four cases, those affected who had tested negative were transmitting the virus to others while their nasal test showed a negative result, according to this work. But what the study does not clarify is whether the detection was better in saliva due to the technique used (PCR), which is more sensitive than antigens, or because saliva is better. sample to diagnose the new variant.
To find out “a broad study would have to be done, with the same type of test, with good sampling, and in more people”, clarifies Dr. Rafael Cantón, spokesman for the Spanish Society of Infectious Diseases and Clinical Microbiology (SEIMC) .
Those who have been on the front line with Covid patients during the pandemic know that not all samples were equally efficient within the respiratory tract. It was changing with the evolution of the disease. Thus at the beginning we saw hospitalized patients who tested negative in the nasopharynx and continued to test positive in samples from the lower tract (low secretions). But to this day there is still “no verified and robust scientific evidence” to say that it is more efficient to do the tests in saliva, clarifies Cantón.
For the virologist Estanislado Nistal, professor of Microbiology at the Faculty of Pharmacy of the CEU San Pablo University, with the evidence we have so far, we can say that the self-tests of antigens, whether nasopharyngeal or saliva, «have similar reliability to detect positives if done correctly». The problem, and one of the reasons why false negatives are sometimes obtained, is that the user does not always follow the steps well. Especially in those who need to insert the swab deep into the nose.
The self-test of saliva antigens, a priori, is easier to perform but you also have to be careful. “If you are secreting a lot of saliva, do not clear your throat or drink a lot of water before, you are diluting it and it may be less sensitive”, warns Nistal.
At the same time, vaccination is also making it more difficult for the virus to cause illness and to replicate better in the upper part of the respiratory tract. «If Ómicron has an easier time staying on high roads, and both nose and mouth are above the windpipe, the positive could be detected both in the nose and in the area around the throat “, explains Nistal.
Thus, we ask the virologist whether the current nasopharyngeal swab tests can also be used with samples obtained from the mouth. «It is a regulatory issue. If you buy an antigen test that has been validated only as nasopharyngeal, it is not proven to be valid for samples from the mouth. In order for it to be reliable, they would have to be approved for that purpose and give the appropriate instructions for use “, clarifies Nistal, who considers that for users who do it at home it would be easier to take a sample from the throat than from the bottom of the nose.
Rafael Cantón is of the same opinion: «Each test is designed and validated for a type of sample. It is not worth using the nasal for saliva until it is proven that it can be used. ‘ Therefore, if someone wants to take a saliva test, they must purchase one that is authorized for it.
On what is the best time to perform an antigen test if there are symptoms, the SEIMC spokesperson recommends doing it as soon as you begin to notice them. If there are no symptoms, but there is close contact with a positive, it is better not to do so until at least 48 hours after the last contact.
In any case, if you are ill, and the antigen self-test is negative, the recommendation is isolate yourself and repeat the test if symptoms persist.
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