What is Patch Testing?
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.
How it Works?
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:
- Erythema multiforme-like
- Discoid (nummular) eczema
- Mucous membrane reactions (conjunctivitis, stomatitis, vulvitis)
- Psoriasis of the hands
- Implant reactions
- Granulomatous or lymphomatoid reactions
- Lichen planus-like
- Certain drug eruptions
- Worsening of pre-existing dermatitis (stasis, atopic or seborrhoeic dermatitis)
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.