Food allergies affect 3 to 8% of the French population. There are different biological tests whose contribution to the diagnosis of food allergy (reliability) depends on their technical characteristics and the circumstances of their prescription.
"The diagnosis of a Food allergies is based on three elements. At least a clinical history compatible with an immediate allergy (within 30 minutes to 2 hours), or rarely semi-delayed (reaction within 1 to 8 hours), skin or biological tests showing an IgE-dependent allergic sensitization or its absence, in the latter case it is the ingestion of the same food that causes the recurrence of the same symptoms that will be retained", introduces Dr. Habib Chabane, allergist in Paris. He also reminds us that the dosage of anti-food IgG is of no use in the diagnosis of allergy or food intolerance.
Food allergies: skin or biological tests
To determine whether a person (adult or child) is sensitized to an allergen, it is possible to do skin tests ( prick tests ) or biological tests (blood test). "Skin tests are positive in the event of an IgE-dependent reaction with immediate manifestations after eating a meal," explains Dr. Chabane, who specifies that allergists prefer to start with these tests. Tests can be carried out with allergenic extracts or a native food if the extract is not available. "The native food has the advantage of containing all the allergens to which the patient may have reacted. However, using the native food does not necessarily make the test more reliable because the extracts are often more concentrated in allergens," specifies the allergist.
The result is visible on the skin after 15-20 minutes. For single-dose food-specific IgE assays, the biologist uses a commercial extract for each food to be tested, if available. The reliability of the assay depends on the quality of the reagent, the robustness of the technique used and the indication for which it was prescribed. "If the skin test or the specific IgE assay is positive for the food tested, there is a strong suspicion of allergy," informs Dr. Chabane. In the majority of food allergies with semi-delayed or delayed manifestations, immediate-reading skin tests and specific IgE assays are negative.
Biological testing for food allergies: a step-by-step approach
If an infant or child up to the age of 5 or 6 has an allergic reaction after eating a complex meal without any suspect food, we look first for sensitization to the most common foods. "Up to this age, 80% of food allergies are linked to 5 or 6 foods . In a probabilistic approach, we will look for the presence of specific IgE using mixtures of 4 to 6 foods (for example, fx5 which contains egg white, fish, cow's milk, peanuts, soy, wheat) and use 2 or 3 different mixtures in order to cover what the child may have eaten. If one or more mixtures are positive, we can then do blood tests of the foods one by one," explains Dr. Chabane, who specifies that it is not because a mixture is positive that we should stop giving the child all the foods in this mixture. It is necessary to continue consuming the foods from the mixture that were consumed and well tolerated after the allergic reaction.
Food allergies: blood tests reliable if interpreted correctly
"These biological tests should not be prescribed systematically, without questioning specifying the foods ingested before an allergic reaction. Their prescription must be imperatively oriented," warns the allergist. If they are done systematically, there is a risk of removing foods that a child tolerates because blood tests are (falsely) positive. "Atopic children can develop a very small sensitivity to a food," explains Dr. Chabane. This does not create a reaction when they eat it.
Blood tests can also be positive because of old sensitization that is in the process of healing. Doing systematic tests on older children and adults poses another problem. People allergic to pollen, for example, can react to plant-based foods such as fruit, or people allergic to mites to shrimp, but there is no point in doing these tests if the foods are perfectly tolerated. "We must therefore always question the relevance of these blood tests and adapt the research to the interview," insists the allergist. "The demonstration of specific IgE by a biological test does not in itself allow a diagnosis of allergy to be established. There must be concordance between this positive test and the clinical history (symptoms, time between ingestion of the food and reaction, etc.) ," he explains. He gives an example: if you have eaten shrimp and within 30 minutes you have swollen up and the blood test is positive, in this case the result is relevant!