Tests That Determine Allergies’ Causes

A number of laboratory tests helps in determining causes of allergies. The tests identify the substance causing allergic reactions / allergens. Commonly used allergy tests include  blood tests, elimination tests and skin tests.

Blood tests are conducted if patients are unable to undergo skin tests. Enzyme-linked immunosorbent assay (ELISA, EIA), a blood test, counts allergen-specific immunoglobulin (Ig) E antibodies found in the blood of the patient. Other examples of blood tests include absolute eosinophil count, immunoassay capture test (ImmunoCAP, UniCAP, or Pharmacia CAP) and radioallergosorbent (RAST) test.

Absolute eosinophil count test calculates the number of eosinophils, the white blood cells, as the cells become active when you have allergies and infections. If the result of a blood differential test is abnormal, absolute eosinophil count test is done. The eosinophil test helps in assessing severity of an allergic reaction. A high count of eosinophil may be ascribed to hay fever, asthma or leukemia. RAST method is used if the result of skin testing is inconclusive or skin testing cannot be done.

Elimination tests use an elimination diet to determine allergies to a food item. The diet means stopping consumption of foods causing allergic reaction for many weeks and re-introducing these food items one by one to observe and monitor allergy symptoms.

The most common allergy test is a skin test that is used to diagnose allergic contact dermatitis, food allergy, penicillin allergy, venom allergy and animal, mold and pollen allergies causing asthma and allergic rhinitis. Skin tests are preferred to blood tests as they are reliable, quicker and cheaper. There are different types of skin tests, such as intradermal skin test, patch testing and prick test.

During intradermal skin test, a small quantity of an allergen is injected into the skin and reaction signs are observed. The test is usually performed to determine whether patient is allergic to penicillin and bee venom. This test is done if a prick test fails. In the prick test, a small quantity of a likely allergen is placed into the skin surface through needle pricks or scratches and reaction to the allergen is watched for up to 20 minutes.

Patch testing means placing allergens on the skin and taping them to the skin for up to three full days. The test is carried out to diagnose contact dermatitis, a skin allergy caused due to chrome found in leather items, nickel used in coins, jewelry and watch buckles, and fragrances.