Skin testing

 

Skin testing is one means of determining whether a person has an allergy to a substance.  There are two types

• The skin prick test
• The patch test

Skin prick test

Skin prick  testing is also known as "puncture testing" and "prick testing" due to the series of tiny punctures or pricks made into the patient's skin. Small amounts of suspected allergens and/or their extracts are introduced to sites on the skin marked with pen or dye (the ink/dye should be carefully selected, lest it cause an allergic response itself). A small plastic or metal device is used to puncture or prick the skin. Sometimes, the allergens are injected "intradermally" into the patient's skin, with a needle and syringe. Common areas for testing include the inside forearm and the back.

 

If the patient is allergic to the substance, then a visible inflammatory reaction will usually occur within 30 minutes. This response will range from slight reddening of the skin to a full-blown hive (called "wheal and flare") in more sensitive patients similar to a mosquito bite.

If a serious life-threatening anaphylactic reaction has brought a patient in for evaluation, then skin tests in general should not be used and a blood test is used instead. Skin tests should also not be used if the patient has widespread skin disease.

The person should also not have taken antihistamines for some considerable days prior to the test.

In general, this test will only identify contact allergens.  The applied substance reach the lower lying immune competent cells [the Langerhahn’s cells) only by passing through the epidermis.  A full immune response cannot thus take place.

 

Thus, for example, this test will not identify heavy metal allergy or the presence of heavy metals, nor will it identify many food allergies or intolerances.  But the test may have a use in determining reactions to dust mites, or animal dander and also nickel in jewellery.

The much deeper intracutaneous test is a great deal more sensitive than the prick test, but is unpleasant and intrusive.  Other types of allergens can be identified, but the problem is that the test is not particularly specific, similar materials can give false positive results

Patch testing

 

Patch testing is a method used to determine if a specific substance causes allergic inflammation of the skin.  It tests for delayed reactions.

It is used to help ascertain the cause of skin contact allergy, or contact dermatitis.

Adhesive patches, usually treated with a number of common allergic chemicals or skin sensitizers, are applied to the back. The skin is then examined for possible local reactions at least twice, usually at 48 hours after application of the patch, and again two or three days later.