A patch test is used to diagnose contact allergy, where a person is allergic to a chemical which is in contact with the skin - for example a person might develop dermatitis on the hands due to allergy to their rubber gloves, or dermatitis on the face due to allergy to their hair dye.

The majority of allergies occur after frequent exposure to the chemical. For example, hair dye allergy is most likely to occur in someone who has dyed their hair numerous times before, without previous problems (but once the person is allergic, then every subsequent time they dye their hair they will react with a dermatitis).

Common allergens include hair colouring agents and bleaches, nickel in jewellery, chromate in cement, a large number of plants, the adhesive used with 'fake' nails, rubber, preservatives in creams and lotions and shampoos, epoxy products.....but there are hundreds of others too.

When a contact allergy is suspected a thorough history is taken, to determine which chemicals (allergens) need to be tested. The test substances are then placed on special chambers on non allergenic tape, and these tapes (each containing 10 allergen samples) are placed on the person's back. Most people having allergy testing done will have 50 or more substances tested.

The tapes are left in place for 2 days during which time the person must not get the back wet. Then the tapes are removed, having carefully marked their location. The sites are checked again in a further 2 days, and again in another 2 or 3 days. In total the test involves four visits.

If a reaction occurs, an area of eczema about 10mm in size develops at the site of the allergen. The severity of the reaction is graded by the therapist or doctor.

A copy of the test results and a report explaining the significance of these results is usually provided to both the patient and the referring doctor.