Patch Testing
This test is performed in cases of contact dermatitis (Eczema) where allergy is suspected.
The allergens are prepared in appropriate concentrations in white soft paraffin (e.g. Vaseline) and are then spread on to discs, 1 cm diameter.
The discs (which are made of a special metal, cannot themselves provoke a reaction) are placed on the skin, usually on the back, and are kept in place by hypoallergenic tape.
The skin is coded appropriately and the patient is asked to keep the skin dry. The patches are left in place for 48 hours.
After 48 hours the discs are removed, the skin is examined and any redness or swellings are noted. The skin is re-examined after a further 48 hours for any remaining local redness or swelling.
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, otherwise the results will be unreliable. Steroid creams need to be stopped for 3-4 weeks before testing as they may suppress the test response.
Any professional interpreting skin, blood or patch tests must first interpret the results in the light of the patient's history. No test should be read in isolation.
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