Doctors diagnose a hives outbreak chiefly by evaluating the patient’s history, performing routine blood tests and, if necessary, performing a skin test to determine the patient’s allergic reactions, WebMD explains. There is no specific test for hives.
The symptoms of hives, or urticaria, include the appearance of red, swollen bumps on the affected area, and a burning, itching or stinging sensation, WebMD notes. Outbreaks of hives may join together, forming an area known as a plaque. In extreme cases, the size of an area affected by a plaque may reach that of a dinner plate.
In a form of hives called angioedema, significant swelling occurs beneath the skin, and the surface layer itself shows no symptoms, WebMD states. The swelling generally takes under 24 hours to fade. Angioedema is likely to appear in areas around the eyes and the lips and less commonly the feet, hands and genitals. Very seldomly, angioedema can appear in the lungs, in the throat or on the tongue, where swelling can block the flow of air and threaten the patient’s life.
When the presence of histamine causes blood vessels in the skin to release plasma, hives or angioedema forms on the skin, according to WebMD. The causes of histamine release include exposure to sunlight, the stings of some insects, medications, allergic reactions and chemicals from some foods. The exact cause of histamine release in a given case is often impossible to determine.