Allergy Testing

It is possible to develop allergy to any substance at any point in time. If you begin to experience symptoms like a runny nose, itching or a rash upon exposure to a substance, you may be having an allergic reaction. Some medications and other substances in food or cosmetics can trigger even life threatening reactions from provoking breathing difficulty.

Symptoms of Allergy

• Reddening of eyes
• Running nose and sneezing
• Itching of the skin at any site
• Skin rash
• Swelling of the lips and face
• Breathing difficulty
• Wheeze and cough

The following allergic disorders can be diagnosed using allergy testing

• Food Allergies like allergy to nuts
• Contact Dermatitis
• Penicillin and other drug allergies
• Allergic Rhinitis (Hay Fever)
• Allergic Asthma
• Dust Allergy

It is important to identify what substances you may be allergic to. This can help in preparing an efficient treatment plan to prevent further reactions. It is recommended to consult with an allergist who is a specialist in evaluating, identifying and treating allergic disorders.

Diagnosing Allergy

A thorough history of the allergic symptoms and its relation to exposure to potential allergens is explored by the physician. Tests are performed to confirm the substance causing allergy.

Allergy Skin Tests

The following tests may be used to identify the source of your allergic reaction.

• Skin Prick Test: In this test, the skin is first cleaned with an alcohol based swab. This is followed by puncturing the skin using a lancet which allows the extract of the allergen  to penetrate into the skin. Allergy to the substance is indicated by the development of an itchy, red, raised bump called a wheal. This appears within about 15 minutes of the skin puncture. Allergy to dust mites, pollen, animal dander and food substances can be tested this way.
• Skin Injection Testing: This is done frequently to test for allergy to penicillin and other antibiotics prior to using them. A small amount of the drug is injected using a syringe and needle into the skin on the fore arm, The area is observed after 15 minutes to look for a wheal.
• Patch Test: This is done to test for skin allergies (contact dermatitis) leading to rash which develop on exposure to of the skin to the allergen. These may be cosmetics, metal or latex. This test is used to detect allergies with a delayed response over days. As the name suggests, a ‘patch’ containing a large number potential allergens is applied on the skin, usually the arm of the back. While the patch is in place, the area should not be exposed to water and bathing is to be restricted. The skin is assessed after 48 hours and the area is examined for sites of irritated skin which would appear red.

Preparing for the test

Keep your physician informed of your current medication list. Certain drugs should be avoided for accurate interpretation of these tests.

• Antihistamines like Claritin, Benadryl and Zyrtec should be stopped about a week prior to performing these tests.
• Heartburn medications like Zantac should also be avoided.
• An asthma drug called omalizumab can potentially affect the results of testing for even a few weeks to months.

Alternate forms of Allergy testing

In some situations it may not be suitable to perform skin testing for allergies. In these situations, blood can be tested for the presence of specific antibodies directed at the potential allergen. These tests are called immunoassays.

Indications for Immunoassay

Life threatening reaction to allergen: in patients with history of anaphylactic reaction to the allergen, this form of testing is preferred as the patient is not exposed to the allergen, only their blood sample is.

Medication Use: In some cases, the individual may be unable to stop taking certain drugs which could disturb the test results.

Skin disease: A disorder of the skin which prevents proper observation and reporting of the skin testing results.

References:

1. Nolte HN, et al. Overview of skin testing for allergic disease. http://www.uptodate.com/home. Accessed Oct. 13, 2018.
2. Kowal K, et al. Overview of in vitro allergy tests. http://www.uptodate.com/home. Accessed Oct. 13, 2018.