What is a Patch Test?

A patch test, also known as an allergy test, is a diagnostic examination conducted to determine if an individual is allergic to certain substances. Performed using a variety of methods, a patch test may cause some discomfort and result in skin irritation if the individual reacts in a positive manner to the applied allergen. There are certain risks associated with a patch test and these should be discussed with a qualified health care professional prior to test administration.

An allergy test is most commonly performed to determine which substances provoke an allergic reaction in an individual. A patch test may be recommended for those whose asthma symptoms are not easily controlled with the use of medication. Individuals who regularly suffer with hay fever may also be given an allergy test. Additional conditions that may prompt the use of a patch test may include food allergies, hives, and penicillin allergy.

There are three main methods utilized in the administration of a patch test. Methods such as skin, elimination, and blood tests are used and all involve the introduction of allergen material to provoke an allergic response. An additional test, known as a provocation or challenge test, involves introducing the individual to a controlled environment where the suspected allergen is either ingested or inhaled. Provocation testing has been known to induce severe allergic reactions and should only be conducted by a qualified health care professional.

Skin tests are the most common method used and may involve a puncturing of the skin and the introduction of a small amount of allergy-inducing substance. Generally administered on the arm or back, a prick test is the most common utilization of a skin test. An intradermal test involves the injection of allergen material and is utilized when testing for specific allergies, such as penicillin. During a patch test, the allergen is placed on the skin and covered with tape for 48 hours, after which time the area will be evaluated for reaction. Skin tests may be used to diagnose a variety of allergies, including those to mold, pollen, and venom.

Elimination tests are utilized to diagnose food allergies and may be conducted over an extended period of time. Known as an elimination diet, one process involves the removal of certain food items from an individual’s diet over a period of several weeks. Once the items have been eliminated from the diet, they are again slowly re-introduced to provoke an allergic reaction. Another approach to the elimination test is called a double-blind test and involves the administration of disguised food items to test for mild allergic reactions. Neither the individual nor the test administrator is aware of which substance is harmless and which is the potential allergen; only a third party is aware of the allergen, which is identified with a code.

Blood tests involve the evaluation of immunoglobulin E (IgE) antibodies once the administration of an allergen has been conducted. Elevated levels of these antibodies are indicative of an allergic reaction. Various other types of blood tests, including a complete IgE level, an absolute eosinophil count, and a serum globulin electrophoresis, are used to evaluate white blood cell and antibody activity in the presence of the introduced allergen.

Prior to any allergy test, a review of the individual’s complete medical history may be performed. Those who are undergoing a skin test are instructed to refrain from taking an antihistamine, which may cause a false-negative reading. A false reading may lead to a potentially dangerous situation if the individual is exposed to a substance for which she or he tested negative and it provokes a serious allergic reaction.

Abnormal results, also known as positive results, to a patch test indicate that the individual is allergic to the administered allergens. The degree of the reaction is dependent on individual sensitivity to the substance. Risks associated with patch tests include severe allergic reactions and anaphylactic shock, a life-threatening allergic reaction that may lead to cardiac or respiratory arrest.