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A single outbreak of hives (urticaria) that resolves without treatment probably does not need a formal evaluation. However, patients with recurrent episodes should see their physician.

No single laboratory test can isolate the specific cause of hives, but there are a number of tests available to identify or rule out certain triggers. A medical history and complete physical examination will provide the best clues as to the cause(s) of an individual’s hives. The physician will try to identify:

• Any allergies a patient or a family member may have
  
• Anyone else in the family or the household who may have hives
  
• New or unfamiliar soaps, detergents, cosmetics and foods the patient may be using
  
• Recent illnesses, both acute and chronic
  
• Types of medications currently being taken or recently stopped
  
• Whether the patient is pregnant or suspects pregnancy
  

Some conditions can have rashes associated with them and may be ruled out before a diagnosis of urticaria is reached, including:

• Joint inflammation. Rheumatoid arthritis produces bumps under the skin.
  
  
• Lupus.  Systemic lupus erythematosus (SLE) is an autoimmune disease of unknown origin that affects the bones and internal organs. Almost half of SLE patients have a classic malar rash (pertaining to the cheekbone) specific to SLE.
  
  
• Respiratory diseases. Diseases of the lung that impair breathing. Microbial infections such as mold exposure and meningitis have skin rashes associated with them.
  
  
• Malignancies. Cancerous growths or tumors. Rashes and other dermatologic conditions can signal the presence of cancer in the body.

Diagnosis of hives is based on the appearance of the skin and, in the case of allergic urticaria, can be confirmed by the history of exposure to allergens. In some instances, skin tests (e.g., allergy skin test, skin biopsy), blood tests or urine tests may be performed to determine which allergens are triggering episodes of hives or to determine other potential causes. Patients may also be instructed to keep a diary for one to two weeks to help pinpoint the trigger. The record should include details such as activities and foods eaten during the period, as well as the timing and location of symptoms.

If food is suspected as the trigger, patients may be placed on an elimination diet. An elimination diet is used to pinpoint and remove foods and ingredients that provoke allergic reactions in a patient. During this process patients eliminate all suspected foods or ingredients from their diets for a period of time. Patients who eventually find themselves symptom-free then gradually reintroduce suspected foods back into their diet, one by one. When a reintroduced food sparks a reaction, an allergy diagnosis is usually made.

However, it should be noted that although the cause of acute hives can often be identified, chronic hives usually cannot be definitively diagnosed. Only about 25 percent of cases of chronic hives are identified, according to the American Academy of Allergy, Asthma and Immunology.