What allergy tests are there?
In allergy tests, a distinction is made between skin tests and laboratory chemical procedures. The skin tests include the following: The tests differ in their invasiveness. In the rubbing test, the allergen (a substance that can lead to an allergic reaction) is rubbed over the inside of the forearm.
In the prick test, an allergenic liquid is applied to the forearm and the skin is punctured with a lancet. The scratch test differs from the prick test in that the skin is first scratched about 1 cm and then the liquid is applied to it. In the intradermal test, the test fluid is injected directly into the skin.
It is also possible to test the blood for certain defence proteins (antibodies, especially IgE), which are formed during an allergic reaction. However, this examination is relatively unspecific. Furthermore, a RAST can be performed, which can detect specific antibodies, but is very time-consuming.
It is also possible to determine the tryptase concentration in the blood. Elevated values can give an indication of particularly severe allergic reactions. – Friction test
- Scratch test
- Intracutaneous test
The prick test is a standard in allergy diagnostics. In this test an area of skin is typically used on the inside of the forearm. At the beginning of the test a grid with numbering is painted on the forearm.
Then drops of various liquids are applied to the skin according to the numbering. A positive and a negative control as well as 15-20 test substances are included in the standard. The positive control contains histamine and always shows a skin reaction.
The negative control is isotonic saline and should not cause a skin reaction. The test substances contain the most common known allergens, i.e. substances to which humans can react allergically. A small prick is made in the skin with a lancet through the drop.
Through this small wound, the fluids reach deeper skin layers. If a person has an allergic reaction to one of the test substances, the liquid is recognised by defence cells, the mast cells. These then release the tissue hormone histamine.
Histamine causes a dilation of the vessels at the skin site. This leads to a reddening of the skin area. In addition, the vessels also become more permeable, allowing fluid to escape into the surrounding tissue. This fluid is then perceived as a small swelling or wheal. Finally, even the smallest nerve endings are irritated by the skin reaction and the typical itching occurs.
RAST stands for Radio-Allergo-Sorbent-Test. This test procedure can be used to assess whether an allergy to a specific allergen is present and how severe the allergy is. In the classical method, cell components (antigens) of a certain allergic substance are first applied to a paper.
In this way, substances to which many people have an allergic reaction or the suspicion of a certain allergen can be examined one after the other. Then some of the patient’s blood is put on this paper. If an allergic reaction occurs, so-called antigen-antibody complexes are formed.
The antibodies are produced by the defence cells in the blood and are defence proteins. They bind specifically to the antigens that have been applied to the paper. These antigen-antibody complexes can be made visible with a radioactive substance.
The amount of radioactive radiation corresponds to the number of antibodies formed and thus conclusions can be drawn about the severity of the allergic reaction. The result is given in RAST classes. 0 means no reaction against the antigen and 4 corresponds to a high dose of antibodies, i.e. a severe allergic reaction. The procedure is rarely used due to the complex procedure with radioactive radiation.