Diagnosis of Rashes

Diagnosis of a simple rash usually involves performing a physical examination of the affected skin and taking a personal and family history, including information about medical conditions (e.g., allergies, asthma).

During the physical examination, the physician evaluates the features of the rash. Features examined include the area(s) of the body affected (called distribution) and the type, shape, and arrangement of the lesions (e.g., patches, blisters [vesicles], papules, pustules, wheals).

A personal history includes information about the following:

  • Additional symptoms (e.g., fever)
  • Exposure to new soaps, detergents, lotions, cosmetics
  • Exposure to wooded areas
  • Medications
  • Recent illnesses
  • Unusual foods

Depending on the suspected cause for the rash, diagnostic tests may be performed. These tests include skin scrapings, allergy tests, blood tests, and a skin biopsy.

Skin scrapings can be used to diagnose some types of fungal or viral infections. These tests involve scraping a sample of cells from the affected area of skin and examining the sample under a microscope. In some cases, the sample is treated with a solution and allowed to dry before examination.

Allergy tests include prick tests, intradermal (i.e., under the skin) tests, and patch tests. Prick tests involve placing small drops of common allergens on the skin (usually on the forearms or back) and then lightly pricking the skin through the drop with a small needle. Intradermal tests involve injecting a small amount of allergen into the outer layer of skin. These tests involve little discomfort and take about 30 minutes to perform.

In patch tests, a small amount of the suspected allergen is placed on the skin (usually on the back) and the area is covered with a bandage. Then, the physician checks for a reaction after 48–72 hours. Patients who have a severe skin condition may require a blood test to make a diagnosis.

Blood tests involve taking a blood sample, adding an allergen to the sample, and measuring the amount of immunoglobulin E (IgE) antibodies produced in response to the allergen.

A skin biopsy involves removing a sample of tissue from the affected area for examination under a microscope. This procedure usually is performed under local anesthesia (e.g., lidocaine) in the physician's office, using a scalpel or a punch instrument, which removes a circular skin sample. Stitches may or may not be required following a skin biopsy.

Publication Review By: Stanley J. Swierzewski, III, M.D.

Published: 01 Jan 2007

Last Modified: 02 Oct 2015