A patch test is used to find the allergic cause of contact eczema and other types of skin reactions.
A range of suspected allergens is prepared in soft white paraffin (e.g. Vaseline) and placed onto a metal disc or strip (about 1cm in diameter). These are then taped to the patient’s back.
The skin is marked appropriately and the patient is asked to keep the skin dry. The patches are left in place for 48 hours. After this time the discs are removed and the skin is examined to see if any red inflamed areas (wheals) have appeared. If no reaction is seen, then another 48 hours (without the patch) is given before the area is re-examined to see if a delayed reaction has occurred.
The interpretation of this form of testing is not as simple as it sounds and tends to be carried out by dermatology departments in hospitals. The symptoms of contact dermatitis need to be brought under control before patch testing can be carried out, or the results will be unreliable. Steroid creams need to be stopped for 3-4 weeks before testing as they may suppress the test response.
Patch testing is often used for the testing of allergic reactions to substances such as:
- Balsam of Peru
- Epoxy resin
- Glyceryl thioglycate (acid perming solution)
- Imidazolidinyl urea
- Paraben mix
- P-tert butylphenol formaldehyde resin
- Rubber accelerators
- Topical corticosteroids
- Wool alcohols (lanolin)
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Last updated: March 2012