The most common form of allergy testing, skin testing for allergies involves introducing a small amount of an allergen to the patient’s skin to determine if the person is allergic to that substance.

Physicians can use skin allergy tests to test for several different allergens at the same time. Skin testing begins with the patient’s skin being marked to identify spots for each allergen to be tested. The testing is often conducted on the forearm, upper arm or the back. Tiny amounts of the allergen are then introduced to the appropriate spot marked on the skin by one of two methods:

• Epicutaneous testing. The allergen is introduced just barely below the surface of the skin by either a prick, scratch or patch method.
  
  
• Intradermal testing. The allergen is injected below the skin with a needle and syringe.

After a period of time, the skin is examined for reactions to any of the allergens. Patches are typically removed at 72 hours. Results from other methods of skin testing are usually apparent within 10 to 30 minutes. If a rash or small bump develops, the patient is most likely allergic to that substance. Generally, the larger the skin reaction, the more sensitive the person is to the allergen.

A skin test should also involve the injection of histamine, a chemical present in cells throughout the body that is released during an allergic reaction. Histamine is considered a positive control because most people have a skin reaction to it, whether they are allergic or not. If a patient does not react to histamine, an allergy skin test may not be useful. In addition, a substance without any allergens in it (e.g., saline) may be injected to see if there is a reaction. If a reaction occurs, it may indicate sensitive skin, which means the results of an allergy skin test should be reviewed with caution.

Allergy skin tests can be safely performed at any age. However, they may not be as useful in children under the age of 3 and patients over age 60. In addition, people with certain skin conditions (e.g., eczema, psoriasis) may yield less accurate results.  In these situations, a blood test that measures allergy-specific antibodies may provide a better alternative for diagnosing allergies.

Some types of medications (e.g., antihistamines, tricyclic antidepressants, corticosteroids) may also interfere with skin allergy testing. Therefore, patients taking these drugs may be asked to discontinue the medications several days before undergoing an allergy test.

In addition, blood tests may be ordered for patients who have a history of the severe allergic reactions anaphylaxis and anaphylactic shock.