Making the Diagnosis
Your doctor will try to identify the trigger by asking when the rash appeared and what you might have done that was different from usual in the days leading up to it.
Blood tests generally reveal very little about hives. In many cases, the cause remains a mystery. It usually doesn't matter, because hives are a brief, solitary event in most people. Your doctor may ask you to keep a record of what you eat and your activities to try to pinpoint the cause.
If you have chronic hives, your doctor will probably examine you for signs of other medical problems that can cause hives to appear, such as lupus and rheumatoid arthritis. Your doctor may ask for routine tests for these conditions.