Slipping a swab into the nostril of a struggling child is now routine for tens of thousands of parents whose children attend kindergarten and primary school. The health protocol that applies in schools – at the time of publishing these lines – indeed implies that as soon as a pupil is tested positive in a class, his comrades must be tested the same day, then two days later, then four days later.
And it is therefore up to the parents to perform this gesture that the youngest have difficulty accepting, sometimes leading them to seek alternative and less invasive solutions. The baby fly technique, for example, which consists of soaking the self-test swab in the mucus collected by the device rather than directly at the bottom of the child’s nostrils.
False negative
Shared on social networks, this method seems to work since many parents report having obtained positive results (which they then had confirmed with PCR or antigen tests carried out by health professionals). But according to Franck Perez, director of the cell biology and cancer unit at the Institut Curie/CNRS, it is far from being without risk.
Indeed, explains the researcher, “naso-pharyngeal self-tests have been authorized or even labeled on validated protocols where we will scrape cells from the epithelium (the wall that lines the inside of the nose, editor’s note), recover them and then test for the presence of viruses in it. If we only recover secretions, the result can be very different”.
Thus, “if we are positive, it means that we are in the infection phase, there is virus released in the mucus and therefore we can see it. There may also be factors in the mucus that reduce the sensitivity of the tests, as the protocols often call for blowing your nose before taking the sample. I don’t fear false positives, but false negatives”.
The return of saliva tests?
Another possible diversion of the self-test: pass the swab inside the cheek to collect saliva. In other words, use this nasopharyngeal test as a saliva test.
Except that here too, the method is risky: the extraction protocols and the virus detection thresholds are not the same, and the risk of a false negative is also extremely high.
In general, recalls the researcher, “a device should not be diverted from the use for which it was designed without rigorously testing this new protocol. What I fear is that improper testing is inappropriately reassuring people and causing them to take more risks.” For themselves and especially for others.
Still, it is difficult to blame parents who are looking for solutions to test their children in more serene conditions. For Franck Perez, the Omicron tidal wave may be an opportunity to give saliva tests their chance again, by carrying out a rapid clinical study: “It’s worth it and it’s more serious than letting people do system D by finding a way not to scratch the back of the nasal wall of their children who can’t take it anymore”.
Adopted and then quickly abandoned because they are less sensitive than nasopharyngeal tests, these tests could well make a comeback. This is what the Minister of Health Olivier Véran suggested in an interview with the Sunday Journal on January 16, citing a preliminary study published in the journal MedRxiv. It suggests that saliva tests would be more effective in quickly identifying contamination by the Omicron variant.
Indeed, it seems that it first appears in the mouth and throat before settling in the nose. Comparative studies are underway, the minister said.