How are skin tests done?
Skin prick test
In this test the doctor puts a drop of watery solution containing allergens such as pollen or trace amounts of animal hair on different marked positions on your skin. A lancet is then used to prick your skin a little at each site so that the allergens can enter the skin. To test for a food allergy, the lancet can first be dipped in the food before being used to prick your skin.
If you are allergic to the substance, your skin will become red or a small bump (wheal) will appear at the marked site. The skin prick test is mainly used to detect allergies that trigger an immediate reaction when they make contact with the skin – such as hay fever or some food allergies.
Intradermal test
The procedure here is similar to that of the skin prick test, but the allergen solution is injected into the skin (intradermal injection) instead. Intradermal tests can also detect weaker allergic reactions.
Because the injections make the test somewhat unpleasant and the injections can also trigger stronger allergic reactions, this test is generally only used if a skin prick test did not show enough of a response.
Skin scratch / scrape test
A skin scratch test can be used if the results of the skin prick test are not clear. A small area of skin is removed and then the allergen is “rubbed” over the skin. This allows the substances to reach deeper layers of tissue than possible in the skin prick test, with the aim of achieving a stronger reaction. The skin scrape test is similar, but only the outermost layer of skin is removed.
In these tests it is not possible to regulate exactly how much of the allergen enters the skin and tissue. They are also more likely to cause non-allergic skin irritation than the skin prick test is. So this test is considered to be less reliable.
Applying the allergen externally
If your doctor thinks that your skin will react very strongly, the substance can simply be rubbed onto your forearm without breaking your skin. Compared to the skin prick test, it will take a little longer for the results to appear: If you have an allergy, the area of skin will turn red and bumpy after about 20 minutes.
Patch test
This test is used if you are thought to have an allergy where the symptoms only become apparent one half to three days after contact with the allergen. The triggers are often single substances found in:
- Medications such as local anesthetics (lidocaine, tetracaine)
- Cosmetics (preservatives and fragrances, or essential oils)
- Jewelry (nickel or cobalt)
- Gloves or condoms (latex)
A patch containing the suspected allergens is put on your back and left on for one full day. If there is no reaction after it is taken off, your skin is checked again after another 24 hours, and sometimes again three days after removing the patch. If you have a contact allergy, your skin will react in the same way it does when it is inflamed (contact dermatitis). The skin becomes swollen, turns red and starts to itch, and small blisters may form.