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A physician will compile a patient's complete medical history and a list of symptoms before making any diagnosis. A family history of the disorder or of allergic reactions is an important clue. A physician is likely to inspect the appearance and location of the rash, to ask how long the rash has been present and to look for evidence of skin thickening (lichenification).

Patients may also undergo allergy testing to ensure that skin symptoms are not being caused by an allergic reaction rather than atopic dermatitis. This may include blood testing (e.g., RAST) or other forms. However, skin allergy testing may not be accurate in people with skin rash-related problems. Some patients may be placed on an elimination diet to determine if the reaction is being triggered by a particular food.

It is not always possible to diagnose atopic dermatitis on the first visit to a healthcare provider. For example, the physician may request that patients eliminate certain foods from their diet or stop using certain detergents or soaps for a period of time. Patients will then have to return to their provider to see if these steps have been effective in controlling symptoms.

A physician will also rule out other potential causes before diagnosing a case of atopic dermatitis. Sometimes, symptoms that appear to be related to atopic dermatitis actually belong to another skin disorder. Such conditions include:

• Seborrheic dermatitis. Commonly known as "cradle cap," it is similar to atopic dermatitis, but differs in that it is not caused by an allergic response.
  
  
• Psoriasis. This is a rash that is caused by the overproduction of skin cells.
  
  
• Contact dermatitis. Skin condition caused by contact with a substance to which the individual is sensitive or allergic.