Stroke the skin of any patient complaining of hives. Most patients with dermographism will form a wheal and red flare minutes later. This establishes the diagnosis. No other tests are required.

Pressure urticaria. Swelling may occur hours after the application of pressure. Many patients with chronic urticaria also have pressure urticaria. Antihistamines may not be effective.

Cholinergic urticaria. Round papular wheals occur shortly after exercise or exposure to heat. These ‘heat bumps’ are very common in young adults.