Coronavirus: a positive self -test with tap water?Why this is not possible

Edit : Ajout le 12 janvier d'une précision sur l'anticorps « coloré»».

A self -test spent under tap water.A few moments later, a positive result is displayed.These are the images and videos that have been circulating for several days on social networks.In videos posted on Facebook on January 8, a man passes the part of the test containing the strip directly underwater.It renews the operation on a second test, this time applying only five drops on the part of the test provided for this purpose.The two tests then appear positive in photos.

The operation aroused many comments questioning the reliability of the tests: "What a scam!"," Eyes powder "," what is this bin’s "...

Fake off

Abbott, the manufacturer of the visible test in these videos and photos, reminds of 20 minutes that his test must be used "with samples collected using a nasal swab inserted into a person's nostrils".The company denounces the "dissemination of false information".

Why did the tests appear positive after being passed under taps?Abbott sketches a track: "Other liquids have chemical properties which can cause a chemical reaction on the reactive strip, resulting in misleading or inaccurate results.»»

Antibodies used

To find out more, we asked Etienne Joly, inserm researcher at the Institute of Pharmacology and Structural Biology in Toulouse. Pour comprendre le phénomène, il faut d’abord se souvenir du fonctionnement de ces tests : ceux-ci utilisent des anticorps, dont un va être « coloré»» et positionné pour pouvoir migrer sur toute la membrane du test.Two others will be fixed at the level of the control and test lines.

When, after having taken his sample in the nose, a person puts his swab in the plastic container which contains liquid, it deposits, if it is infected with the COVVI-19, virus proteins in this liquid.The swab must normally have collected it if the person carries the COVVI-19.

Viral proteins are deposited on the strip

A quoi sert ce liquide dans lequel il faut faire tremper l’écouvillon ? Il « contient des quantités importantes de détergent, et de sel, qui permettent de dissocier les particules virales, afin que chacune des protéines soit solubilisée»», rappelle Etienne Joly.

Then, under the normal conditions of use of a test, the person who tests pays a few drops of this detergent on the strip.There, the solubilized viral proteins will combine with the colored antibody and will migrate on the test strip like water on a blotter.

They will then meet the other antibodies present on the test and control lines, which reveals these two famous lines.

Pourquoi ces lignes apparaissent-elles, donnant un faux négatif, lorsque de l’eau du robinet est versée sur un test ? Serait-ce parce que cette eau contiendrait un très grand nombre de particules virales ? Le chercheur écarte cette hypothèse, « très peu probable»».

The most likely hypothesis comes from the misuse of these tests and the absence of detergent: "These tests are based on the solubilization of viral particles in a environment containing detergent," recalls the scientist.If you use tap water, the sample you deposit will not contain detergent, and the antibody combined with the colorful reagent will tend to fix in a non -specific manner at the level of the test strip.»»

Other videos with Coca-Cola, Beer or Compote

En août, alors que des vidéos similaires étaient devenues virales, Akli Bouaziz, directeur scientifique des laboratoires AAZ, un fabricant d’autotests, dénonçait également un « mésusage»» auprès de nos confrères de France 24 et de l'AFP : « Le fait de dériver le test et de mal l’utiliser déclenche ces deux bandes où le test semble " positif " mais il ne l’est pas.If you take the test correctly used with the reagent, you add a drop of water, the test will be perfectly negative [in the absence of viral particles].The reagent is absolutely necessary for the proper functioning of the test.»»

In other videos, Internet users had paid Coca-Cola, Beer or even Compote on PCR or Antigenic Tests.There too, these experiences did not demonstrate that these tests detect the COVVI-19 virus in these liquids.