DECRYPTION – While temperatures have been dropping steadily since the beginning of January, antigen tests carried out outdoors can generate false negative or positive results.
The thermometer continued to fall during the month of January and the barnums of tests continue to flourish in front of pharmacies. An uncomfortable situation for those who take the samples, but also a headache in terms of test results. Thursday, January 27, the spokesperson for the Union of Community Pharmacists (USPO), Gilles Bonnefond, warned against the cold, which could alter the proper functioning of antigenic tests such as self-tests, and generate false positives. “There are two instructions to follow“, he explained to our colleagues from the Parisian . “Tests should not be stored in the cold […] and afterwards, when you do them, you have to do it above 7 or 8 degrees“, he detailed.
As a reminder, the antigen test, which is faster and less sophisticated than the PCR test, is carried out by nasopharyngeal sampling, using a swab. This sample will then be mixed with reagents and the solution will be applied to a strip which changes color depending on whether or not the virus is present.
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A temperature window to respect for the analysis
The problem does not come from the levy itself, which “can be done at a lower temperature“, at 7 or 8 degrees, as the pharmacist specifies in the Ile-de-France daily. A finding shared with the Figaro by Professor Marie-Edith Lafon, head of the virology department at the Bordeaux University Hospital. “I don’t see why the sample would be affected by the cold if it’s done well: it goes to the bottom of the nasal cavities, which are far from 0°C, even if it’s very cold outside“, she says, pointing out that “the viruses are in any case very well preserved by the cold“.
Low temperatures, on the other hand, can play a role in the “to analyseof the test. The strips and reagents, most often stored outside under barnums, and not inside pharmacies, can indeed be affected by certain temperatures. This step “must be carried out at room temperature to have perfect operation“, insists Professor Bruno Lina, virologist and member of the Scientific Council. “On all the test boxes, it is indicated a temperature range in which the analysis must be made, generally between 5 and 25°C“says the specialist. Result: If the temperature is too low or too high, “there can be skewed results, in both directions“.
Although each manufacturer of tests – 194 are approved in Europe – has its own instructions for use, the principle is generally the same for everyone. “When it goes below 0°C, the reagent will start to freeze, and the speed of migration of the liquid on the strip will be modified.“, explains to Figaro Akram Yahia Ammar, research and development manager at Biosynex, the French leader in the testing market. With a risk of generating false positives. For temperatures between 0 and 4°C, it is the “reaction kinetics” which will be modified, namely the “ability of the antigen to recognize the antibody“. False negatives may then appear. Furthermore, the “humidity factor» can also have an impact according to the manufacturers. “It all depends on how we developed and evaluated the test“says the specialist.
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A phenomenon already observed last winter by the DGS
This experience of distorted antigen test results had already been observed during the past winter. “Several reports from health professionals have shown the occurrence of false positives for rapid antigenic tests, in particular when these are carried out outside the walls (in a tent, barnum or ventilated room)“, had indicated the Directorate General of Health last January, in a note to health professionals. Contacted by Le Figaro, the DGS did not communicate the number and proportion of false cold-related tests reported from the field. But she says that “the guarantee of the performance of a test and of a reliable and secure result is linked to strict compliance with the conditions of storage, conservation or use, provided for in the instructions“.
Self-tests, sold by the millions in supermarkets for several weeks, are impacted in the same way. It is necessary “pay attention to storage conditions“, warns Gilles Bonnefond, with the Parisian. “Logistics in mass distribution is not always the same as for pharmacies. Users should also take care to keep their tests at room temperature.“, advises the pharmacist.
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