A GP can usually diagnose contact dermatitis from the appearance of your skin and by asking about your symptoms.
They'll want to know when your symptoms first appeared and what substances you've been in contact with.
If a GP has diagnosed contact dermatitis, they'll try to identify what has triggered your symptoms. If the allergens or irritants can be identified, you can take steps to avoid those substances and reduce the risk of your symptoms flaring up.
A GP will look at your medical history and ask questions about your lifestyle and occupation. They may also ask whether there's a history of dermatitis or eczema in your family.
If the allergens or irritants causing your contact dermatitis cannot be identified, you may be referred to a dermatologist (a doctor who specialises in treating skin conditions).
You may also be referred to a dermatologist if the trigger has been identified, but your symptoms are not responding to treatment.
The best way to test for a reaction to allergens is by patch testing. During a patch test, tiny amounts of known allergens are applied to your skin.
The substances are attached to your back using a kind of non-allergic tape. They may sometimes be attached to the upper arms.
After 2 days, the patches are removed and your skin assessed to check if there has been any reaction.
Your skin will usually be examined again after a further 2 days, as most allergic contact dermatitis reactions take this long to develop.
It's difficult to test whether specific products irritate your skin, because testing for these is unreliable.
In some cases, a repeated open application test (ROAT) is useful, particularly to assess cosmetics. A ROAT involves reapplying the substance onto the same area of skin twice a day for 7 days, to see how your skin reacts.
This is a particularly useful way for you to check your own cosmetics at home for reactions.