Patch tests are essential for determining if a patient’s condition (e.g. dermatitis) is caused or aggravated by a certain contact allergen.
A wide range of substances can be used for patch testing. Series of allergens (e.g. nickel, parabens, fragrance perfume, rubber antioxidant) are applied to every patient. 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 any allergic reactions.
The detailed report on allergens is completed after the second and third appointments which are usually 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 are described as 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 one has, and the nature of the allergen.
It is not recommended to swim, rub or exercise because patches can easily come off. The back has to be kept dry, which means the back area 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:
Skin inflammation (dermatitis) can be caused by various factors including Inherited as well as chronic by nature, triggered by irritant substances, or triggered by allergic reactions. Dermatitis triggered by allergic reactions (Allergic Contact Dermatitis (ACD), appears on the skin of people who have developed a Contact Allergy.
Contact Allergies are usually developed over time by exposing the skin to certain substances. Once, if the contact Allergy has been developed, the skin will have dermatitic reaction each time during the contact with the contact allergen, also called hapten. In order to determine which substance or product may cause the skin inflammation, the Patch Test is performed.
The Patch Testing comprises application of the different allergens (haptens) into Patch Test Units which are taped onto the skin on the back. In people with Contact Allergy, the Patch Test will cause some redness and inflammation that represent on a small scale the allergic reactions. This type of skin reaction is usually delayed and develops over 2-4 days, which requires subsequent return to the clinic for the assessment and interpretation of the result.
Patch tests are not the same as skin prick tests, which are used to diagnose other type of the allergic reaction, for instance, hay fever allergy (house dust mite, grass pollens and cat dander), or hives. Skin prick tests are not diagnostic for patients with dermatitis.
Diagnosis of Contact Allergy when Allergic Contact Dermatitis (ACD) resulting from type IV hypersensitivity is suspected regardless of anatomical site of dermatitis. Indications also include conditions that may mimic a contact allergic reaction and need to be ruled out, such as:
Patch testing requires few appointments within a one-week of the test. Usually, after the test application on Day 0, 2 or rarely 3 appointments are needed for readings. On the day 2, the Patch Test Units are removed, and the main reading is performed within 48 hours after – day 3 or day 4. In certain situations, a delayed response reading may be performed a week after the initial test application for delayed response reading.