Itching and rashes may develop as the result of infection or irritation or from a reaction of the immune system. Some rashes occur mostly in children (see Symptoms in Infants and Children: Rashes in Children), whereas others almost always occur in adults. Sometimes an immune reaction is triggered by substances a person touches or eats, but many times doctors do not know why the immune system reacts to produce a rash.
The diagnosis of most noninfectious rashes is based on the appearance of the rash. The cause of a rash cannot be determined by blood tests, and tests of any kind are rarely performed. However, persistent rashes, particularly those that do not respond to treatment, may lead the doctor to perform a skin biopsy, in which a small piece of skin is surgically removed for examination under a microscope. Also, if the doctor suspects a contact allergy as the cause, skin tests may be performed (see Diagnosis and Treatment of Skin Disorders: Skin Tests).
Last full review/revision December 2006 by Peter C. Schalock, MD