What is it about ?
It can be difficult to make the correct diagnosis in skin diseases. Most often, the doctor bases his diagnosis on the discussion (the history) and on the appearance of the lesions. Many of the lesions associated with skin diseases are similar, and often these lesions are due to an underlying condition.
This is the reason why, over the years, a number of skin tests have been developed alongside blood tests, to facilitate or confirm the diagnosis of skin abnormalities, mainly of an allergic nature.
What blood tests?
IgE (for immunoglobin E) are antibodies produced by the body in contact with foreign substances or allergens (substances to which the person is allergic). These antibodies can be detected and measured in the blood. Sometimes you test for a whole group of substances, for example if you are allergic to different types of trees. In other cases, the test carried out is “specific”, which means that it detects antibodies against one and only one allergen, for example in case of allergy to cats.
The detection of IgE is mainly useful in theatopic eczema in children if we suspect a form offood allergy. It can also be very useful in the presence ofurticaria and D’drug allergy.
The presence of antibodies in the blood does not necessarily mean that a person is actually allergic. It only indicates a predisposition to allergy. The association of respiratory or skin symptoms with the detection of antibodies in the blood naturally points very strongly in the direction of an allergy.
These blood tests are not influenced by allergy and / or itchy medication. These drugs can therefore be taken normally on the day of the examination.
What skin tests?
In principle, skin tests are carried out by a doctor specializing in the skin (dermatologist).
Skin allergy test (prick test)
A skin allergy test (prick-test) consists of piercing the skin with a thin needle, then depositing a droplet of solution containing the allergen to be tested so that it comes into contact with the blood. If the person is allergic, a red pimple (papule) with itching usually appears within 15 minutes. The test is positive when the papule is at least 3 mm in diameter. The larger the diameter, the more severe the allergy.
It is possible to test at the same time a whole series of allergens (trees, plants, animals, dust mites, …). Each test requires a different prick in the skin.
The dermatologist also does two control tests: a neutral control test, which the person should not normally react to, and a histamine control test, which everyone reacts to for sure.
Antihistamine medications for allergies and / or itching should be stopped about 5 days before testing.
Patch tests
Patch tests consist of sticking an adhesive containing allergens on the skin, most often on the back or on the upper arm. The adhesive is smoothed for 2 days before being removed. It may take up to 5 days for the rash to appear. A positive reaction results in the appearance of a red spot on the skin. This spot is measured.
Generally, it is possible to detect contact allergies using a standard test comprising about 30 chemicals. But there are also specific tests, such as for cosmetics, glues, hair spray, etc.
Antihistamine drugs or a small dose of oral cortisone do not affect the tests. However, a cortisone-based cream can weaken the reaction.
Photo-tests
Photo-tests are used to detect skin diseases related to the sun (photodermatoses or photodermatitis). These tests are only carried out in special cases. They consist of administering increasing doses of UVA and UVB rays to the skin. The doctor checks to see if a rash appears. Sometimes the reaction is delayed for a few days.
Photo-patch test
The photo-patch test is carried out like an ordinary patch test (see above), but the allergens are applied in duplicate. After 2 days, the patches are removed. Half of allergens are exposed to sunlight, while the other half are not. This test aims to check if a contact allergy can be triggered by sunlight (photoallergic contact eczema).
Physical urticaria tests
These tests are used to detect hives created by pressure (dermographism), cold, heat or sweat. Dermographism is the phenomenon characterized by the appearance of a raised line (accompanied by itching) on the skin immediately after rubbing or scratching it. Cold urticaria is detected by applying ice cubes to the skin; heat urticaria, by applying a tube of hot water to it. For the sweat test, the person must exert effort on a treadmill.
Provocation tests
The main purpose of provocation tests is to detect allergic contact dermatitis. They are designed to trigger an allergic reaction by administering an allergen to the skin or through the mouth.
For the open application test, the suspected allergen is lightly rubbed onto the skin of the forearm. The result is visible after 15‒20 minutes.
For the iterative open application test, the suspected allergen is applied to the skin, for example twice a day for 7 days. The reaction usually develops between the 2th and the 4th day.
For the oral challenge test, the person swallows the allergen. The main use of this test is to see if the person is allergic to foods or drugs. If the person has had severe reactions in the past, this test should be done in the hospital.
For the subcutaneous challenge test, a small amount of the allergen is injected under the skin. This test is performed in case of suspected allergy to certain drugs, local anesthetics for example. The reaction is generally observed after one hour.
Want to know more?
- Allergological assessment (patch tests or prick tests), here, Where to find on this page of Cliniques St Luc UCL
- The skin allergy test explained to children and in pictures – plaster
- Antihistamines – CBIP – Belgian Center for Pharmacotherapeutic Information
- Corticosteroids for the skin – CBIP – Belgian Pharmacotherapeutic Information Center