In the diagnosis of allergic diseases, there is the special problem that the symptoms of the disease – such as rhinitis, asthma or eczema – are confronted with a vast number of allergenic substances. In order to find the right allergen for the patient from the at least 20,000 scientifically known allergens, complex diagnostic methods are sometimes required. Allergy diagnostics usually proceeds in four stages.
1st allergy diagnostics: anamnesis.
Valuable information about the possible allergy trigger can already be obtained from the collection of the allergological disease history (anamnesis). In addition, the home and work environment, lifestyle and dietary habits, and at least an orienting assessment of the psychosocial environment should be considered. In particular, self-observed relationships between allergic symptoms and possible allergens as well as the environmental conditions of a private and professional nature should be recorded. Particularly important are the recording of the onset of the disease (including possible “harbingers”) and evidence of primary allergen contact.
2. skin testing
Skin tests (prick test, intradermal test, scratch test, and rub test) are a foundation of allergy diagnosis. Here, samples of various substances (possible allergens) are applied to the skin and observed to see if an allergic reaction occurs at that skin site (as a pustule or wheal). Depending on the diagnostic objective, the physician may limit himself and test by individual samples the allergens suspected according to the previous questioning (“confirmation test”). In most cases, however, this is a search diagnostic, in which it is important to cover as broad an allergen spectrum as possible in a single session by means of group extracts. Skin tests lead to false results if antihistamines or corticosteroids are taken at the same time. Therefore, antihistamines should be avoided as early as five days before an intended skin test. These tests are not ideally suited for young children.
- Prick test: a drop containing the test solution is applied to the arm and then the skin is pierced at this point with the prick lancet to a depth of about 1 millimeter. In the case of an existing allergy to the test substance, a wheal will have formed at this point after about 20 minutes. For allergies of the immediate type, the prick test is used as a standard method.
- Intradermal test: Here, the allergen is injected into the skin with a needle. The intradermal test is about 10,000 times more sensitive than the prick test, but more often gives false positive results, especially for food allergens.
- Scratch test: through the applied test solution, the skin is scratched superficially. Because of the relatively large skin irritation, this test is not always clear. Therefore, the scratch test has nowadays lost its importance.
- Rub test: The allergen is rubbed back and forth several times on the inside of the forearm. This test is used when there is a high degree of sensitization of the patient. Since this test is performed with the natural allergen, it is also suitable if the allergenic substance is not available in industrially prefabricated version.
- Epicutaneous test (patch test): patches with allergen-containing substance are applied to the skin (preferably back) and read after 24, 48 or 72 hours. This test is used to identify type IV allergens.
3. laboratory tests
In blood tests, blood samples are used to determine the reactivity and specific sensitization to the allergens under investigation in the laboratory. One criterion is the presence of specific IgE antibodies. Modern methods are used to measure the amount of antibodies (immunoglobulin E) in the blood. Immunoglobulin E is formed in response to the foreign substances to which the immune system of an allergy sufferer reacts sensitively. Blood tests are therefore particularly suitable for allergy diagnostics in babies and young children, as they are less stressful for the young patients, as only one blood sample is required. Above all, there is no risk to the child even in the case of severe hypersensitivity. In addition, taking medication does not affect the result, whereas skin tests can be falsified by this. Finally, the doctor can even predict the probable allergy career of his patient on the basis of the results of the laboratory test and often prevent worse with appropriate countermeasures.
4. follow-up history and provocation test.
Interpretation of the test result always requires verification by obtaining a “posttest history” (Has the patient been exposed to the allergen at all? Do symptoms and test result match?). Whether the IgE antibody determined by positive skin tests and/or blood tests corresponds to a current clinical efficacy of the allergen in question can only be definitively clarified by direct testing on the organ in question using a provocation test. Provocation test: In the provocation test, the clinical symptom (for example, conjunctivitis with redness and eye tearing, asthma, skin rash, eczema) is reproduced by mimicking the “natural” allergens to a large extent.
