Allergy testing - Investigations and procedures

Allergy testing

Allergy testing most commonly involves skin prick tests and may include the radioallergosorbent test (RAST) and rarely diagnostic allergen challenge. The choice of test is dependent on the suspected allergen and the nature of any previous allergic reaction. Testing of patients with a history of anaphylaxis may be confined to RAST and should only be undertaken by an allergy specialist.

Skin prick tests allow the testing of multiple allergens simultaneously. Small amounts of a specific suspected allergen or a panel of common allergens is applied to the skin, which is then pricked to allow the allergens to penetrate the skin. An erythematous reaction followed by a weal occurs within minutes when positive. The

test is read at 15 minutes and a weal diameter of ≥3 mm is normally considered positive. Skin tests are useful in detection of respiratory allergies, food allergies and allergies to penicillin and insect bites. Patients should not be taking antihistamine medication at the time of the test.

RAST measures specific IgE antibodies in the blood to individual allergens.

Food allergies can normally be diagnosed on a good clinical history. If doubt exists skin prick tests or RAST tests (usually if there is a history of anaphylaxis) can be helpful. True identification may require food challenges with the patient blinded to the food being tested, as there may be a psychological component to presumed food allergies.

Other tests include immunoglobulin and complement measurements and blood eosinophil counts.