Patch test (medicine)

A patch test is a method used to determine if a specific substance causes inflammation of the skin. It is often used to test for allergens.

Any individual with suspected allergic contact dermatitis and/or atopic dermatitis needs patch testing. The first appointment will take about half an hour. Tiny quantities of 25 to 150 materials (allergens) in individual square plastic or round aluminium chambers are applied to the upper back. They are kept in place with special hypoallergenic adhesive tape. The patches stay in place undisturbed for at least 48 hours.

At the second appointment, usually two days later, the patches will be removed. Sometimes further patches are applied. The back is marked with an indelible black felt tip pen or other suitable marker to identify the test sites.

These marks must still be visible at the third appointment, usually two days later (4 days after application). The back should be checked and if necessary remarked on several occasions between the 2nd and 3rd appointments.

Interpretation of the results

The dermatologist will complete a record form at the second and third appointments (usually 48 and 96 hour readings). The result for each test site is recorded. The system we use is as follows:

Negative (-)

Irritant reaction (IR)

Equivocal / uncertain (+/-)

Weak positive (+)

Strong positive (++)

Extreme reaction (+++)

Irritant reactions include sweat rash, follicular pustules and burn-like reactions. Uncertain reactions refer to a pink area under the test chamber. Weak positives are slightly elevated pink or red plaques. Strong positives are ‘papulovesicles’ and extreme reactions are blisters or ulcers. The relevance depends on the site and type of dermatitis and the specific allergen. The interpretation of the results requires considerable experience and training. If the result(s) considered positive, the person is probably allergic to that substance(s).

Common allergens

The most frequent allergen that was recorded in many research studies all around the world is Nickel. Nickel allergy is more prevalent in young women and it is especially asoociated with ear piercing or any nickel-containing watch, belt, zipper or jewelry.