Which Tests Are Most Important In Determining The Causes Of Chronic Hives?

The evaluation of chronic hives should always begin with a thorough medical history and physical examination; however, in most cases a cause will not be identified. Most physicians will order a general blood panel, including a complete blood count and white cell differential, erythrocyte sedimentation rate, chemistry panel, thyroid panel, and urinalysis, in order to rule out major systemic diseases. If all of these tests are normal, as they most often are, a number of more specialized tests may be considered.

First, and most importantly, is a test called the autologous serum skin test. As the name implies, blood is taken from the patient and centrifuged to separate the blood cells from the serum; a very small amount of the patient’s own serum is then injected intradermally and observed for 15 minutes. If an allergic reaction develops at the site of the injection, it is good evidence that the patient has antibodies in the blood that are attaching to components in the skin and causing the mast cells to release histamine and other chemicals. This procedure is typically done only by an allergist with extensive experience in treating chronic hives.

As an alternative to this skin test, a commercial blood test has been developed (Chronic Urticaria Index, IBT Labs), which will detect whether the patient has autoantibodies directed against components in the skin. If the results of either the blood or skin tests are positive, it confirms the diagnosis of autoimmune hives. If the patient finds that hives or angioedema consistently follow eating, and particularly if the symptoms are more episodic in nature, then allergy skin or blood testing to foods may be useful. In most cases, the foods that are selected for testing are based upon the results of a food diary , in which patients record all episodes of skin lesions along with all foods eaten during a 2- to 4-week period.

Those foods that were eaten less than 4 hours before hives would be selected for testing. If patients have daily hives and/or angioedema, however, with no obvious temporal connection to eating, then allergy testing to foods is rarely useful. Patients who have isolated angioedema, particularly those who have had throat or abdominal swelling, should be considered as having possible hereditary angioedema, and special tests of the complement system (C4 complement component and C1 inhibitor functional assay) will need to be performed.