A rash (allergic contact dermatitis) from poison ivy, oak, or sumac plants usually is diagnosed through a physical examination and questions about your outdoor activities. Your health professional will examine the rash to decide which kind of plant caused it and ask about:

• The length of time between possible exposure and development of the rash.
• The length of time you were out in the sun.
• What you were doing when the exposure may have occurred (hiking, for example).
• Other rashes you have had.
• Your occupation and hobbies.

Diagnosis is harder when there are no clues that you have been in contact with the plant. Sometimes indirect contact with the plant oil (urushiol) causes the rash. Urushiol that is on clothing or other objects may spread to your skin months after initial contact. If a bacterial infection is suspected, your health professional may take a sample of blister fluid for a culture.

If your primary care doctor is not sure of the cause, you may need to see a dermatologist for a direct patch skin test. In this test, small amounts of allergens are placed on the skin and covered with a bandage for several days. The patch is then removed and the skin examined for reactions.