Screening for allergies is necessary in order to identify the specific allergens that trigger reactions in sensitive individuals. By identifying the allergens, an allergy sufferer can avoid the offending substances as much as possible to reduce the frequency of episodic attacks, take medications to relieve symptoms, premedicate when exposure is likely, or undergo immunotherapy. Just as there are many types of allergic reactions, varying in symptoms and severity, there are many types of allergy tests.
Types of Allergy Tests
After taking a medical history and performing a physical examination, the doctor decides whether an allergy test is necessary, and if so, which one. In some cases, more than one test may be administered.
Blood tests screen for allergies by detecting antibodies produced by the body's immune system. The enzyme-linked immunosorbent assay (ELISA), which measures the amount of immunoglobulin E in the blood, is the most common blood test administered. Blood tests are useful in finding a wide range of allergies, but are considered most helpful in diagnosing food allergies. Blood tests may also show an elevated level of a certain type of white blood cells (eosinophils) that are indicative of inflammation.
IgE skin tests, also known as scratch tests, are very useful in diagnosing allergic triggers. During these tests, a tiny drop of a solution containing the potential allergen is placed on the skin and the skin is scratched or pricked to allow the allergen to invade the body.
After about 20 minutes, the skin is checked for an abnormal reaction, usually a small swelling similar to a mosquito bite. If the patient's skin swells at the point of contact, it indicates that the allergen applied to that spot is the trigger. Allergy skin tests are commonly administered to diagnose:
- Allergic rhinitis
- Food allergies
- Allergies to bee venom
- Allergies to latex or penicillin
Patients may show an allergic reaction to many of the applied substances or to only one. If a hive does not appear on the patient's skin surface, but the doctor is still suspicious, a more sensitive intradermal test, in which the allergen is actually injected, may be administered.
Skin tests are typically performed in the allergist's office to ensure proper results and to decrease the risk in case of a rare side effect or severe reaction.
Challenge tests are normally administered to diagnose an allergy to a specific food or medication. During challenge tests, a small amount of the potential allergen is either inhaled or taken orally. It is essential that challenge tests be supervised by trained allergist so appropriate measures can be taken in case of a severe reaction.
In some instances, patients are instructed to stop taking antihistamines before an allergy screening in order for results to be accurate. In other instances, however, especially those in which the patient has had severe allergic reactions in the past, the patient may be required to take an antihistamine before the test to mitigate a possibly acute reaction.