Dermatological tests to detect certain types of allergies
Patch tests are essential for determining if a patient’s condition of dermatitis is caused or aggravated by a certain contact allergen. It is important to know that patch tests are not the same as skin prick tests. Skin prick tests have a very limited value for patients who experience skin rashes and this testing method is manly used for diagnosing hay fever allergy (grass pollens, house dust mite, and cat dander).
A wide range of substances can be used for patch testing. Usually, European Standard Series of allergens (e.g. nickel, parabens, fragrance perfume, rubber antioxidant) are used by majority of dermatological clinics and these allergens are applied onto patient’s back. Each substance has to be applied in an appropriate amount to have the best concentration in order to demonstrate an allergic reaction without causing any irritation to others who are not allergic to this material.
Sometimes results can be misleading or inconclusive. In some cases, instead of an average one or two positive reactions, almost all test areas become red and start itching. In dermatology, this condition is referred to as “angry back” which is usually caused due to an active form of dermatitis. Moreover, sometimes there may be not enough reaction to a substance that regularly causes dermatitis, which is known as the false-negative result. Lastly, further testing may be needed for some patients because patch tests may not necessarily explain the cause of dermatitis.
All appointments take approximately 30 minutes. During the first appointment, tiny quantities of 80 standard materials in individual plastic squares or round aluminum chambers are applied onto the upper back. For extra stability, it is suggested to tape them with a special hypoallergenic adhesive tape. The patches have to stay undisturbed and immovable for 48 hours.
During the second appointment, further patches may be applied or completely removed. The back gets marked with an indelible black tip of the marker to identify the sites of testing. On the third appointment, marks have to be still visible. The back gets checked for presence of any allergic reactions.
The detailed report on allergens is completed after the second and third appointments which usually include 48 and 96-hour readings. The results for each test site are recorded based on the following classifications: Negative (-), Irritant reaction (IR), Equivocal/uncertain (+/-), Weak positive (+), Strong positive (++), and Extreme reaction (+++).
Irritant reactions cause sweat rash, follicular pustules, and burn-like reactions. Uncertain reactions appear as pink areas on the back. Weak positives appear as slightly elevated pink or red plaques. Strong positive reactions are “papulovesicles” and extreme reactions contain ulcers or blisters. The relevance of allergic reactions depends on the site of the exposure, type of dermatitis a patient has, and the nature of the allergen.
It is not recommended to swim, rub, or exercise because patches can easily come off. The back area has to be kept dry, which means that it should be prevented from unnecessary sweating and contact with water (e.g. shower and bath). No products (e.g. lotions, steroid creams, body oil) should be applied on the back for the entire duration of patch testing.
Positive patch test results are small areas of active dermatitis/eczema. They may be itchy and may require treatment with prescribed topical steroids. Patch test reactions may remain persistent for several weeks. Patch tests may provoke other areas of dermatitis to reoccur or even appear for the first time.
Centre for Medical and Surgical Dermatology offers patch testing as part of dermatitis treatment. For more information on patch tests, visit the following link:
