ALLERGY PATCH TESTING

Edited by, SNEHA MAVIS

Allergy patch testing is a method of screening substances to determine the source of an allergic skin reaction. Patches containing various suspected allergens (irritants) are applied to a person's back and left in place for 48 hours. After that, the skin is inspected for symptoms of hypersensitivity reaction.

WHAT ARE THE SUBSTANCES TESTED?

Hair dye, preservatives, cosmetics and pharmaceuticals are among the allergens examined in an allergy patch test. Almost every patient is exposed to a standard set of allergens, such as those outlined in the European standard series of allergens, because these are the most common causes of dermatitis in around 85% of cases. A variety of other compounds are also used which are frequently customized to the patients. To this purpose, the patient may bring their own materials from home or work to see if these are triggering their symptoms.

TESTING SCHEDULE:

The following is the general format for the patch test.

Day 1: plastic or spherical aluminium square champers containing small amounts of each substance are applied to the patient's upper back and attached with hypoallergenic tape.

After 48 hours: the patches are removed and the skin's reactivity is assessed.

After 96 hours: a second assessment is carried out and evaluations are made.

In some cases an additional assessment may be required after 7 to 10 days.

RESULT:

The interpretation of skin reactions needs a significant amount of experience and training. It is critical to differentiate between irrational and an allergic reaction. The former happens immediately after the patch is removed but fades over the next day, whereas an allergic reaction develops over several days and was most prominent on day five rather than immediately after patch removal.

The following types of reactions may be noticed during an allergy patch test

   • Negative

   • Irritant reaction such as a burn-like rash or follicular pustules.

   • Uncertain due to the existence of a pink area under the test chamber.

   • Weakly positive - slightly elevated pink or red plaques 

   • Strongly positive - characterized by papules or vesicles.

   • Extreme - shown as blisters or ulcers.

A positive reaction results in an area of itchy, active dermatitis near the patch application site. Topical steroids are typically used to treat this condition.

Medications can potentially interfere with patch test results. Different medicines can either suppress or raise the risk of developing allergic reactions, reducing the test's accuracy. Antihistamines, tricyclic antidepressants, and the asthma medicine Omalizumab are examples of these medications. Patients may be required to stop medicines for up to 10 days before to a patch test.