Because there are so many possible causes for urticaria, other cases require determined detective work on the part of the patient and physician. A single episode of uncomplicated urticaria does not usually need extensive testing.
An episode of hives complicated by swelling or trouble breathing requires immediate evaluation in the emergency room.
In many cases, the cause of chronic hives cannot be identified despite detailed testing; in this instance the condition is called idiopathic urticaria.
The most common reason for long-lasting urticaria is dermographism.
These hives appear within a few minutes of scratching along an area of skin.
Sun-induced urticaria may occur within a few minutes after exposure to the sun.
Certain types of chronic hives are more painful than itchy.
An episode of hives may last from a few minutes to several hours to several days to several weeks.
Each individual welt should last no more than 24 hours. Acute episodes of urticaria last for six weeks or less.
Medications such as aspirin and antibiotics (especially penicillin and sulfa) also are common causes of hives.
Infections causing hives include the common cold, strep throat, infectious mononucleosis and hepatitis.
Cold urticaria (from exposure to low temperature followed by re-warming) can be severe and life threatening if there is a generalized body cooling, for example after a plunge into a swimming pool.
Cholinergic urticaria is due to an increase in body temperature with sweating, exercise, hot showers and/or anxiety.
Most often, the rash is linear, following the path taken by the act of scratching.