Several steps are needed to diagnose an allergy. The first one is the anamnesis, or patient history – a discussion with the doctor during which the patient describes his or her symptoms, when they occur, and how severe they are.
Using the patient health questionnaire, the patient answers questions about whether he or she is on medication, what the living conditions are, and whether there are any hereditary conditions. Sometimes it is useful for the patient to keep a symptom or food diary as part of this process. In some cases, a preliminary suspicion can be established, but the anamnesis is not sufficient for a diagnosis. Therefore additional tests need to be performed. The symptoms and other factors determine which test is suitable and how many tests must be conducted.
Sensitisation tests such as skin tests or blood tests reveal whether the body is predisposed to an allergy to certain substances. A skin test is usually performed for confirmation. Blood tests are particularly useful for cases such as when skin testing is impossible due to the condition of the skin. If the cause of the symptoms cannot be determined using these methods, or if it remains unclear whether the test results are at all relevant for the patient, the doctor can perform a provocation test.
It is often assumed that blood and skin tests are able to provide clear evidence of an allergy, but this is not the case. They provide information about an allergic event in the body. Only testing combined with doctor-patient discussions and provocation testing, if applicable, can lead to a clear diagnosis.
A patch test is usually performed in order to diagnose contact allergies to substances such as nickel chloride in costume jewellery or perfumes in cosmetics. Contact allergy symptoms often only occur after a certain period of time following exposure to the allergy-triggering substance. For this reason, the test involves placing specific allergens into test chambers and applying them to the skin with adhesive tape, where they are left in place for (24 to) 48 hours. The test results are read after (24), 48 and 72 hours. The doctor checks whether an eczema reaction has occurred, and if so, the progression and severity of the reaction.
The prick test is used to test for immediate-type allergies. In this type of allergy, the reaction usually occurs just a brief time after contact with the allergen and can progress to a very severe degree. Pollen, insect venom and most food allergies are examples of this type of allergy. In the prick test, allergen extracts are applied to the skin with a dropper. The skin is then pricked lightly with a small blade without drawing blood. The results are obtained after 15 to 20 minutes by evaluating the size and severity of the redness on the skin.
The prick-to-prick test is very similar to the simple prick test; however, real food is used instead of commercial allergen solutions. The lancet is first pricked into the food and then into the skin.
In the intracutaneous test, the allergen solution is not applied to the skin but is instead injected into the surface of the skin. This test can also be used to reveal immediate-type allergies. Since this test involves a high risk of a severe allergic reaction, it can only be performed under close medical supervision.
Some substances, such as latex or animal dander, can trigger an immediate allergic reaction through simple contact with the skin. In such cases, the suspected substance is rubbed on the skin or applied to the skin. The condition of the skin is then assessed after 20 minutes.
IgE Blood Test
Allergists conduct a blood test to determine the level of IgE antibodies in the blood. These are specific antibodies that the body produces for its own immune defence. Total IgE is measured in order to determine whether more antibodies are being produced overall. This can occur when an allergic disease is present, but also in the case of a parasitic infection. An IgE test can also be used to determine whether more antibodies are present that target a special allergen source, for example, specific IgE antibodies against birch pollen. The doctor usually also checks the ratio of total IgE to the IgE value for the suspected allergen.
The IgE blood test does not provide information about whether symptoms are present and which ones, nor does it predict which symptoms may develop in the future. A representative study conducted by the Robert Koch Institute revealed that nearly half of all adults in Germany are sensitised to at least one substance. On the other hand, the percentage of people who have already been diagnosed with an allergy once during their lifetime is around 30%. The reasons behind this discrepancy have not yet been investigated. Despite its limitations, the IgE test is and remains an important tool for doctors in determining the causes of allergies.
IgG Blood Test
The body produces different types of antibodies. In addition to IgE antibodies, there are also IgG antibodies. Allergists determine the level of IgG antibodies in order to find out whether a patient suffers from certain inflammatory respiratory diseases (Farmer’s Lung, for example).
Home testing for IgG food sensitivities is available from certain online providers. This allows patients to find out themselves whether they have a food allergy. From an allergological standpoint, IgG tests are not suitable for confirming food allergies. This is because an increased level of IgG indicates that the body has come into contact with a substance and has developed a tolerance. It is inadvisable to avoid certain foods due to increased IgG levels as this can lead to nutrient deficiencies.
The provocation test is particularly useful when blood and skin tests have failed to provide conclusive results as to whether a substance is triggering a patient’s allergy symptoms, and if so, which one. In this test, the patient is given a miniscule amount of a substance and then observed for any physical reaction. There are three different types of provocation tests.
Oral Provocation Test
The oral provocation test uses foods to provoke a reaction. Strict safety precautions and constraints are taken during this test, which may only be performed by a doctor. A hospital stay is usually required for this test. Patients are given the food in the form in which it is normally eaten. Physical reactions are observed in order to assess which foods trigger allergic reactions and need to be avoided in the future. An important objective of the provocation test is also to determine which foods do not cause any symptoms and can be consumed without concern in the future.
Nasal Provocation Test
In the nasal provocation test, the suspected allergen (birch pollen, for example) is applied to the nasal mucous membrane of hay fever patients. The patient is then observed for nasal irritation, reduced nasal inspiratory flow, and/or increased mucous production.
Inhalative Provocation Test
In order to confirm or exclude the presence of an allergy in patients with a respiratory disease, the bronchial mucosa can be exposed to the suspected allergen (certain types of pollen, for example) through inhalation. Pulmonary function testing is then used to measure the effect of the suspected allergen on the patient’s breathing.
Prof. Dr. med. Dr. h.c. T. Zuberbier
Last changes made: July 2016