Patch testing is undertaken to determine whether somebody has a delayed allergic response to a contact allergen – i.e. something coming into contact with their skin which results in contact eczema or dermatitis. Often within a few hours of coming into contact with the allergen the affected skin will become itchy and inflamed.
Nickel is the primary confirmed cause of allergic reactions worldwide. It is found in jewellery and coins. Other common allergens are contained in cosmetics, toiletries and fragrances and for some people, contact with rubber will cause an allergic response. In other cases medicaments, such as creams, may give rise to contact dermatitis. Long term exposure to these allergens results in people becoming increasingly sensitised to them.
Some forty or fifty standard substances which are frequently in contact with the skin will be tested. These include rubber, perfume, preservatives, metals and plants. The consultant dermatologist will have discussed each patient’s history with them and may test for other substances with which they come into contact at work or at home.
Following your initial consultation with the consultant, you will be given three further appointments within one week, often on a Monday, Wednesday and Friday.
At the first appointment a specialist nurse will apply the tests to your back in small containers held together by a tape and the names of each will be marked in ink. Itching following this is normal but is not necessarily indicative of an allergic reaction.
At the second visit the patches will be removed and any reactions will be noted by the consultant, who may then ask for additional patches to be applied.
On the third visit, the consultant will examine your back and discuss the results with you. Positive findings will guide future treatment and advice but negative findings will be helpful too, since they can shift the focus for the skin condition away from contact allergy as a cause.
Overall, patch testing is very safe and there is no risk of infection associated with it. It is, however, contraindicated while patients are taking a moderate to high dose of steroids. It is also considered inadvisable for patients to undergo patch testing within two weeks of sunbathing or exposure to sun beds or sun lamps. It is not generally undertaken when patients are pregnant or breastfeeding.