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To diagnose contact dermatitis, a physician will perform a complete physical examination and compile a thorough medical history. Special attention will be paid to the patient’s history of exposure to an allergen (substance that triggers an exaggerated immune system response) or irritant and the appearance of the skin. Patients are questioned about their daily activities, health, hobbies, workplace responsibilities and use of medications and cosmetics.

The appearance of the rash on the body aids in diagnosing contact dermatitis. Contact dermatitis eruptions often have clear-cut margins, geometric outlines and acute angles that appear anywhere from a few minutes to a few days after contact. Because allergic contact dermatitis (ACD) and irritant contact dermatitis (ICD) skin rashes start out looking the same, it is not always easy to determine whether an allergen or an irritant is the cause of contact dermatitis. If the itchy rash and redness give way to blisters that form a crust or ooze, the condition is more likely to be diagnosed as allergic dermatitis, such as from poison ivy.

The location of the rash may also help the physician pinpoint the substance causing the dermatitis. For instance, contact dermatitis on the neck may be caused by perfume or cologne.

If a case of contact dermatitis is mild and responds well to any initial treatments, testing may not be necessary. However, more severe cases require laboratory testing to identify the responsible substance. Two tests used to diagnose the condition include:

• Patch testing. Allergy testing that introduces a suspect allergen to the skin to determine if an allergic reaction occurs. It often is performed on patients who have chronic, recurring contact dermatitis.
  
  
• Skin lesion biopsy or culture. Examines a sample of skin taken from the site of irritation. Although this test does not reveal the actual allergen, it aids diagnosis by helping to rule out conditions that look similar to contact dermatitis (e.g., psoriasis, fungal infections).

A physician can use a specialized test, known as a photopatch test, to diagnose photocontact dermatitis.