The diagnosis of contact dermatitis is based upon your full story of how and when the suspected substances affected your skin and an examination by the doctor. If symptoms improve after the offending substance is removed, this supports the diagnosis.
Patch testing may be recommended where the cause is unclear or symptoms are recurrent. This test may be performed by a dermatologist or allergist. On the first visit, small patches of possible allergens are applied to the skin. These patches are removed 48 hours later to see if a reaction has occurred. A third visit, about 2 days later, is done to look for any delayed reaction.
If you have already tested a material on a small area of your skin and noticed a reaction, you should bring the material with you.
Other tests may be needed to rule out other possible causes, including skin biopsy or culture of the skin lesion.