Like any disease, the diagnosis of allergy begins with careful assessment of the patient's clinical history and symptoms. Allergy testing may then be used to identify the specific causes of the allergic reaction. There are three common testing methods.
In Vitro (Blood) Test
Today's in vitro allergy testing technology permits physicians to diagnose allergy directly from a blood sample with minimal intrusion. The test quantitatively measures the immunoglobin E (IgE) antibody associated with allergic response, is compatible with all categories of allergens, and is the preferred method for diagnosing food and drug allergies. The blood test does not require patients to discontinue use of antihistamine or other allergy medication, and eliminates the risk of a severe reaction during the testing process.
Advantages: Minimally intrusive method, physician diagnosis, improved sensitivity for food and drug testing.
Disadvantages: Typically higher cost and turnaround time for results.
Skin Test
Traditionally, a patient is tested for allergy by applying a small quantity of allergen to a puncture in the skin and measuring the size of the reaction that develops. Skin tests may be performed by either the skin-prick method or the intradermal method to obtain qualitative results. In both methods, a needle is used to deliver each allergen to the skin; however, the skin-prick method is applied on the upper layers on the skin, while the intradermal method requires injection the deeper layers of the skin. With both types of testing, the size of a reaction on the skin is monitored (the "wheal" and "flare"). While skin tests are very sensitive, they are not very specific, and may generate false positives under a number of conditions.
Advantages: Fast turn-around of results, highly sensitive, and cost-effective.
Disadvantages: Patient discomfort, incidence of false positives, and potential for interference from antihistamines, other drugs, and skin conditions. Procedure is not standardized and measurement of results is subjective.
Provocation or Challenge Test
The provocation or challenge test is often used to assess food allergies. In this test, the patient ingests samples of various foods in capsule form and he or she is monitored for any reaction. In the best-controlled tests, neither the patient nor the physician knows which food is in each capsule. Capsules of non-reactive "placebo" may be tested as well; such studies are called double-blinded placebo-controlled food challenges. Provocation tests must only be performed in the presence of medical personnel who are trained to handle patients who have severe allergic reactions.
Advantages: Eliminates subjective conclusions based on use of placebo.
Disadvantages: Time-consuming test, complicated experimental design, and administrators may not notice delayed reactions.
