What kinds of allergy tests are there?

Skin tests are easy and quick to do, so they are often the test of choice for determining what is causing an allergy. Possible allergy triggers are applied to the skin and the reactions are observed. This is most commonly done on the inside of your lower arm, or sometimes on your back.

In this test, the doctor puts drops of different watery solutions containing allergens (like pollen or trace amounts of animal hair) on various marked positions on your skin. Your skin is then pricked a little at each site so that the allergens can enter the skin. To test for a food allergy, the food can be pricked first – before pricking your skin.

If you're allergic to the substance, your skin will react: On light-colored skin, the marked site will become red and a small bump (wheal) will appear. Wheals develop on dark skin too, but they're more the color of the surrounding skin or a bit lighter. 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.

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.

A skin scratch test can be used if the results of the skin prick test are not clear. This involves removing a small area of skin and then "rubbing" the allergen over the skin. That way, the substances reach deeper layers of tissue than possible in the skin prick test, with the aim of provoking a stronger reaction. The skin scrape test is similar, but only the outermost layer of skin is removed.

In these tests, it’s 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.

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, a reaction will be visible after about 20 minutes. Light skin will then become red with swollen bumps (wheals). Wheals develop on dark skin, too, but the affected skin is darker in color rather than red.

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 a similar way to when it's inflamed (contact dermatitis): It will become swollen and itchy. Light-colored skin usually turns red as well. This is less common and less noticeable in dark skin. Dark skin is more likely to become thicker and rougher in the affected area. Small bumps and dark spots may develop too.

To find out what is triggering an allergy, a blood test may be needed too. It is considered if, for example, a skin disease may influence the results of the skin prick test, or if a skin test would cause too much of a reaction due to a known severe allergy. Sometimes a blood test is used to confirm the results of a skin test that didn't provide clear enough results.

Blood is taken from a vein in the arm for this test. The blood is sent to a laboratory, where the number of a specific type of antibodies – the IgE antibodies – is measured. If someone has an allergy, they may have more of these antibodies in their blood. But this test is only a sign of an allergy, and not proof. Higher numbers of IgE antibodies can also be caused by other things, such as smoking or a parasite infection. The amount of these antibodies does not necessarily correspond to the severity of the symptoms, either.

Someone who has allergy symptoms may not have a strong allergic reaction on their skin. Then a provocation test can be used to help find out what the trigger is.

To see if you have hay fever, for instance, an exact amount of the allergen thought to be triggering it – such as certain types of grass pollen – can be applied to the mucous lining of the nose. Then the doctor can observe whether and how strongly the body reacts with sneezing, a stuffy nose and watery eyes. Allergens can also be tested in a similar way on your eyes or lungs. Provocation tests are usually only done under medical supervision because they can cause very severe allergic reactions.

The search for an allergy trigger follows a sequence of steps: A skin test is usually done first, once you have talked with a doctor. A blood test is done instead if a skin test is too difficult or risky, or if the allergen in question isn’t available in a solution that can be used in skin tests. Provocation tests are only considered if the skin test or a blood test haven’t provided clear results.

Before you have a skin test, it is a good idea to tell the doctor if you have any medical conditions affecting your lungs or skin, or if you have any cardiovascular (heart or blood vessel) problems. It is also important to mention any medication you are taking that may affect how your skin reacts, such as the antibiotics amoxicillin or penicillin, or painkillers like acetylsalicylic acid (the medication in drugs like “Aspirin”) or ibuprofen.

You should have no noticeable acute allergic symptoms right before doing a provocation test. Your doctor should be ready to respond quickly to any strong allergic reactions that may arise during skin tests, and particularly during provocation tests.