Definition
The prick test is a frequently performed skin test that is used to determine a hypersensitivity reaction to certain substances. A so-called type 1 allergy (immediate type) is determined.
When is a prick test performed?
The prick test is always carried out when there is a suspicion of a so-called type 1 allergy in someone. To this type of allergy belong very different allergies, which are sometimes widespread in the population. Therefore the prick test is often used.
These include, for example, allergies to insect venoms, such as bees and wasps, but also many food allergies (nut, soy, shellfish, etc.) and the widespread pollen allergy. Many people are familiar with the prick test from the diagnosis of allergic asthma or hay fever (allergic rhinoconjunctivitis), as it is often used there in particular.
In principle, however, it can also be carried out in other situations on suspicion. For example, this could be a suspicious reaction after eating food or an allergic reaction after an insect bite. Drug allergies are excluded. Although these often also belong to the so-called type 1 allergy, one usually refrains from such a test and simply switches to a tolerated drug. This topic might also be of interest to you: Allergy test
Prick test for hay fever
Hay fever is also known in medical terminology as allergic rhinoconjunctivitis. There are various allergens, such as pollen, which, when inhaled, cause symptoms. Since hay fever is a type 1 allergy, a prick test can be used to diagnose and identify the allergens. However, for small children, a blood test is preferable to the prick test, as it is better tolerated.
Evaluation of the prick test
In a prick test, the sensitization to a certain substance, also called allergen, is tested. For this purpose, up to 20 test solutions with different allergens can be applied to the skin using a lancet. To be able to evaluate the test correctly afterwards, a negative and a positive control must be applied.
The positive control contains histamine, which causes a reaction in the form of a wheal. The negative control contains saline solution and does not cause a skin reaction. After about 20 minutes the prick test can be read.
The physician assesses whether there is a positive reaction based on the appearance of the skin. He compares each test solution with the positive and negative controls. If a wheal has formed on a test solution, this is considered “positive”.
The positive result indicates that the test person shows sensitization to this substance, for example pollen. To assess the strength of the reaction, the diameter of the wheal is measured. The positive reaction ultimately only indicates sensitization and no allergy.
An allergy can only be spoken of when symptoms are also present. To illustrate this, an example: The prick test shows a sensitization to pollen. If there are complaints during the time of the pollen flight, for example in the form of hay fever, an allergy to pollen is present.
The numbering in a prick test has the task of being able to assign the individual skin areas to the applied liquids. Normally, 15 to 20 different allergenic liquids are dripped onto the forearm during the prick test. These liquids are all transparent, so that it is not possible to distinguish between them.
Therefore, each liquid has a number so that in case of a positive skin reaction, the allergen can be traced back to the liquid. The size of the wheal does not correspond to the actual severity of the allergy. Thus, strong skin reactions in the prick test can be accompanied by an only mildly pronounced allergy.
On the other hand, it is also possible that a certain allergen only shows a small skin reaction, but can be very damaging in everyday life. Even if the prick test is negative, an allergy may be present. The prick test shows a sensitization to various test solutions containing common allergens.
Therefore the prick test covers different, frequent allergies. However, the test cannot cover every conceivable allergen. Therefore, an allergy can also be present in the case of a negative result.Furthermore, the prick test is only suitable for the diagnosis of a so-called type 1 allergy.
However, there are other forms of allergy for which other test procedures, such as the epicutaneous test, are used. An example for such an allergy would be an allergic contact eczema, as it occurs with a nickel allergy. If a certain allergy, for example to house dust mites, is suspected despite a negative prick test, the so-called intradermal test can be performed. It is somewhat more precise than the prick test and is particularly suitable for “weak” allergens such as house dust mites, which often do not cause a positive reaction in the prick test.
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