A patch test is used to find allergic causes of eczema/dermatitis and some other types of skin reactions such as reactions to drugs.
Your doctor may refer you for a patch test:-
Every patient is tested to the common chemicals known to cause allergy (such as nickel in metals, fragrances & other cosmetic ingredients, rubber chemicals found in gloves etc) together with additional allergens related to your specific problem e.g. work chemicals or extra ingredients of cosmetics or treatments. Each chemical is applied to a disc (about 1cm in diameter and then taped to your back in strips of 10). Each patient may be tested for allergy to up to 100 different substances. You will also be asked to take a small sample of any material that you think is causing a reaction (such as a cosmetic) to be tested as well.
The hospital will need normal skin to apply the tests so any eczema on your back will need to be under control. Ideally, to avoid suppressing a reaction steroid creams should not have been used on your back for 3-4 weeks before the tests and you should not have had a lot of sun exposure. High doses of steroid tablets (prednisolone more than 10mg) may also interfere with the results.
Your skin is marked to identify the location of the tests and you will be asked to keep the skin dry. The tests shouldn't prevent you from going to work. The patches are left in place for 48 hours. After this time, the tests are removed and the skin is examined to see if any reactions (red itchy areas) have appeared. Additional tests may be applied to help confirm doubtful reactions and it may be necessary to shine light on some of the tests if a sun induced reaction (eg to sunscreen) is suspected. A final reading is taken after a further 2 days and the results discussed with you.
If an allergy is found and it is established that it is contributing to your eczema avoidance should result in a lasting improvement in your eczema. Even if the tests are negative this can be helpful in reassuring your doctor that we are not missing an allergic cause and that treatments given should be effective at controlling your skin disease.