If an allergy is suspected, an allergy test can be done to identify or rule out a specific allergen. Allergy tests can be divided into skin tests, blood tests and provocation tests.
Skin tests should be performed solely by trained personnel. Interpretation of an allergy test is the most difficult part and requires experience. Whereas a simple blood sample has to be collected for the blood test, the patient in a provocation test needs to be kept under medical observation.
There is no totally reliable test method for diagnosing allergies, which is why the experience of a specialist is required. Any allergy test must be preceded by a thorough interview (history-taking) to obtain information about symptoms, the disease course of the allergy, the family incidence of allergies and previous treatment methods. The allergy tests are interpreted in conjunction with all this information.
A distinction is made between skin tests to detect immediate-type and delayed-type reactions. Skin tests are relatively simple to perform and are not painful. When detecting an immediate-type reaction, various allergen solutions are dropped onto the skin of the forearm and pricked with a fine needle (prick test) so that the allergen (protein) comes into contact with the skin cells. The test reaction is assessed after 20 minutes. A positive reaction looks like a mosquito bite. With delayed-type reactions, a patch is fixed onto the back and the reaction (usually a small area of eczema) is assessed after one or two days.
The blood test is done in the laboratory, where the blood is tested specifically for antibodies (specific IgE) to individual allergens. The doctor interprets the results alongside the patient’s medical history. The blood test can be performed at any time and regardless of whether the patient has already taken any medication (e.g. an antihistamine). There are more than 500 different allergens available for testing. These are always a mixture of different allergen fragments (epitopes). Some of these epitopes are highly allergenic, others less so. It is possible to test for individual proteins of a whole allergen (e.g. birch pollen). Knowledge of an individual’s sensitisation pattern is a decisive factor when planning a specific immunotherapy because it means the prospects of success with a desensitisation programme can be assessed more accurately. At the same time, it can help to identify a cross-reaction, e.g. if allergic reactions to hazelnuts and apples are known in addition to a pollen allergy. For people with food allergies, the sensitisation pattern may sometimes provide information about the severity of the reactions.
In a provocation test, the allergen is administered either orally, by inhalation or by direct contact with the skin or mucous membrane. This test should only be performed by specialised doctors. With this form of provocation, the symptoms are monitored and triggered while the patient is kept under observation.
Other terms for desensitisation are: hyposensitisation, specific immunotherapy or more recently allergy immunisation. The aim of desensitisation is to teach the body to again accept those foreign substances (proteins) to which it has reacted with an allergy (hay fever, asthma). The repeated administration of constantly increased doses of an allergen solution induces the immune system to tolerate these foreign substances. In most cases, desensitisation prescribed by an allergy specialist brings about an improvement in allergy symptoms. However, this form of treatment only lasts three to five years.