In the diagnosis of allergic diseases, there is the specific problem that the disease symptoms - such as runny nose, asthma or eczema - face a plethora of allergy-causing substances. In order to find out from the at least 20, 000 hitherto scientifically known allergens that applies to the patient, it sometimes requires complex diagnostic methods. The allergy diagnostics usually runs in four stages.
1. Allergy diagnosis: history
From the survey of allergological disease history (anamnesis) can already be gained valuable information on the possible allergy trigger. In addition, the home and professional environment, the living and eating habits and the at least orientating coverage of the psychosocial environment should be taken into account.
Particular attention should be paid to self-observed relationships between allergic symptoms and possible allergens, as well as the environmental conditions of a private and occupational nature. Especially important are the recording of the onset of the disease (also possible "harbingers") and the indications of the primary allergen contact.
2. Skin tests
Skin tests (prick test, intracutaneous test, scratch test and rub test) are a foundation of allergy diagnostics. Here samples of various substances (possible allergens) are applied to the skin and it is observed whether an allergic reaction occurs at this skin site (as a pustule or wheal). Depending on the diagnostic objective, the physician may restrict himself and use individual samples to check the allergens suspected by the previous survey ("confirmatory test"). In most cases, however, it is a search diagnosis that requires group spectra in a single session to cover the broadest possible allergen spectrum.
Skin tests lead to false results when antihistamines or corticosteroids are taken at the same time. Therefore, antihistamines should be avoided as early as five days before an intended skin test. For toddlers, these tests are not optimal.
- Prick test: Apply a drop of the test solution to the arm and then pierce the skin at this point with the Prick lancet approximately 1 millimeter deep. In the case of an existing allergy to the test substance, a wheal has formed after about 20 minutes at this point. For immediate-type allergies, the prick test is used as the default method.
- Intracutaneous test: The allergen is injected into the skin with a needle. The intracutaneous 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 on the surface. Because of the relatively large skin irritation, this test is not always clear. Therefore, the scratch test has lost importance today.
- Rub test: The allergen is rubbed several times on the inside of the forearm. This test is used when there is a high level of patient sensitization. Since this test is carried out with the natural allergen, it is also suitable if the allergenic substance is not available in industrially prefabricated version.
- Epicutaneous test (patch test): Plasters containing allergenic substance are stuck to the skin (preferably back) and read after 24, 48 or 72 hours. This test is for the identification of type IV allergens.
3. Laboratory tests
Blood tests are used to examine reactivity and specific sensitization to the allergens in the laboratory using blood samples. One criterion is the presence of specific IgE antibodies. Using modern methods, the proportion of antibodies (immunolubolin E) in the blood is measured. Immunoglobulin E forms in response to foreign substances to which the immune system of an allergic person is sensitive.
Blood tests are therefore suitable for allergy diagnostics, especially for babies and small children, because they are less stressful for the little patients, since only a blood sample is required. Above all, there is no danger even if the child is very hypersensitive. In addition, the intake of medication does not affect the result, while skin tests can be falsified thereby. After all, the doctor can even predict the likely allergy career of his patient based on the results of the laboratory test and often prevent worse with appropriate countermeasures.
4. Post-anamnesis and provocation test
The interpretation of the test result always requires a review by collecting a "post-anamnesis" (is the patient exposed to the allergen, do the 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 respective allergen can only be definitively clarified by direct testing on the relevant organ with the aid of a provocation test.
Provocation test: In the provocation test, the clinical symptom (for example conjunctivitis with redness and eye tears, asthma, skin rash, eczema) is reproduced by extensive imitation of the "natural" allergens.