Food allergies and the path to diagnosis

Food allergies can only be diagnosed by a doctor, and the path to diagnosing a food allergy can be a complicated and lengthy one. It begins with a patient recounting their medical history. The physician will ask for relevant information, such as what symptoms of a potential food allergy have occurred, and whether other family members suffer from allergies. Keeping a diary of food consumed and respective symptoms experienced can be very helpful here.

Additional steps to diagnosing food allergies

Blood and / or skin tests are the next steps. In the blood of a food-allergy sufferer, the number of certain antibodies is elevated. This so-called specific IgE antibody value can be determined very well. However, a blood test alone does not provide definitive results. Only through the results of the blood tests (i.e. the IgE antibodies) and detailed symptom reporting can a food allergy be diagnosed or ruled out.

The standard skin test for food allergies: the prick test

With a prick test, the allergist marks spots on the skin on the inside of the forearm. There, he places a drop of solutions containing the allergens to be tested. Then, the allergist applies the liquid into the skin, which absorbs the allergens. Under the surface, the allergens are in contact with so-called mast cells. If an allergy is present, the mast cells emit messenger substances such as histamine, causing the skin to redden and swell into hives; the patient may also experience some itching.

False positives from the prick test

Test reactions can only provide clues and do not reflect the true extent of an allergy. Drugs such as antihistamines, sleeping pills, or cortisone can dampen the reaction. Irritated skin, on the other hand, can cause a particularly strong test result even if the allergic reaction is only weak in everyday life. In addition, fever, infections, and alcohol consumption can falsify test results.

This testing is further complicated by the fact that a prick test does not exist for all food allergens. If, for example, a pepper allergy is suspected, no such testing liquid is available. In this case, the allergist could carry out a prick-to-prick test. He first pricks the food which is suspected of triggering the allergy (for example, into a pepper) and then into the skin of the patient.

Only half of all positive test results match the symptoms.

As with the positive blood test, a food allergy is only present when the test results match the patient's symptoms. As a rule of thumb, the medical guideline for food allergies states that only half of all positive test results on allergies really come with symptoms.

The next step for diagnosing a food allergy: elimination diet.

If blood and skin tests are positive, the next step can be a diagnostic diet. This is selected according to the test results. If there is a reasonable suspicion of individual allergy triggers – such as chicken, eggs, or soy – the diet will consist of removing these potential allergens from the diet. For a certain period of time, the patient has to abstain from these foods completely.

After a few weeks, the patient’s symptoms should improve significantly. If the improvement fails to materialize, this means there is likely no connection between the symptoms and the particular food. 

When a provocation test is useful - and when it isn’t.

If, however, the dietary change leads to an improvement, the doctor and the patient decide whether a provocation test is necessary to confirm the diagnosis. The potentially allergen is administered and an allergic reaction is maintained. Only the provocation can confirm or exclude the connection between symptoms and nutrition with certainty. In individual cases - if the results so far are clear - the doctor and the patient can also decide against a provocation test.

Further reading:

Food allergy symptoms

Causes of food allergies

Preventing food allergies

Food allergy treatment