Dry red inflamed skin of elbow: eczema
A rash is a difference in the appearance of the skin. Rashes can affect the whole body or just one or a few areas. They can be itchy or not. Some rashes are painful (for example shingles). Some, such as chicken pox or ringworm, are due to infection. Others are caused by inflammation, such as eczema, psoriasis or urticaria (hives). Some rashes are due to allergy, for example allergy to nickel in a watch strap causing contact dermatitis. Rarely, cancer in the skin can have the appearance of a rash.
An experienced dermatologist can often make a diagnosis from the appearance and symptoms of the rash. Many rashes consist of characteristic lesions (marks on the skin) in a particular distribution (the body sites affected). Tests may be needed to make or confirm a diagnosis. These can include gently scraping the top layer of the skin to check for fungal infection, taking a swab for bacterial or viral infection, or taking a skin biopsy (a small operation that involves numbing the skin with local anaesthetic before surgically removing a sample of skin for examination under the microscope). Patch tests are used to identify the cause of allergic contact dermatitis. Blood tests are sometimes needed; patients with persistent itch in the absence of a rash may be deficient in iron or vitamin D, or may have an underactive thyroid.