Dermatologists use a variety of tests and assessments to perform a rash evaluation. The type of test your dermatologist performs depends on your symptoms and health history.
Your dermatologist may perform a patch test to determine if your rash is the result of allergic contact dermatitis. In this test, your dermatologist places small patches, which look like adhesive bandages, on your skin. The patches contain several different substances that are likely to cause skin reactions. Your dermatologist will recommend that you wear the patches for 48 to 96 hours before returning to the office, where your dermatologist will remove the patches to look for signs of irritation.
Rash evaluation may include blood tests to check for infections. Your dermatologist may recommend a biopsy, in which they remove a small area of skin for evaluation in a medical laboratory.