Your doctor will ask you about your medical history, including your history of allergies and your work history, to check for possible exposure to chemical irritants or to people with infections. Your doctor also may ask specific questions about your rash, including:
When it began — Did the rash appear after you ate a new food, tried a new skin care product or took a new medication?
Location and pattern — Does the rash affect only sun-exposed areas or only areas in direct contact with gloves, shoes, goggles or face masks (as would be expected with allergic reactions to a chemical in the item)? Does it form a "butterfly" pattern over the cheeks and nose (a classic sign of lupus), or does it produce a bright red "slapped cheek" pattern (a sign of fifth disease)? If you are a hiker, does it form linear streaks along the lower legs (a sign of poison ivy)?
Duration — Did the rash appear and disappear within a day or two (as in roseola), or has it lasted for a week (as in fifth disease) or longer (as in SLE)?
Occupational exposures — Are you a day care worker who may be exposed to children with rash-producing illnesses (measles, rubella, roseola, fifth disease)? Do you work or play near wooded areas where there is an increased risk of tick bites?
Your doctor may suspect a specific cause based on your medical history and the history of your rash. Your doctor will try to confirm this suspicion by examining the rash's appearance, location, pattern and any associated symptoms. In many cases, the results of your physical examination will clarify the diagnosis, and no further tests will be needed.
When required, additional testing may include:
Blood tests — Although most viral rashes do not require specific identification of the virus, blood tests are available to identify some viruses and bacteria that cause rash-producing infections. Blood tests also may be done to check for autoimmune disorders.
Patch tests — If your doctor suspects a local allergic reaction, he or she may conduct skin tests called patch tests. In these tests, tiny amounts of various chemicals are placed on your skin for two days to see if an allergic rash develops.
Wood's lamp — A Wood's lamp is a black light used to help evaluate rashes. Depending on the specific reason for the rash, the light may cause the affected area of skin to glow red, pale blue, yellow or white.
Tzanck test — In this test, a blister is opened and scraped to obtain a sample that is checked in a laboratory for signs of herpes virus infection.
KOH preparation — In this test, a skin area that is suspected of having a fungal infection is scraped gently. Scraped material is placed on a slide, treated with KOH (potassium hydroxide) and examined under the microscope for signs of fungi.
Skin biopsy — In this procedure, the skin is numbed and a sample of affected skin is removed and examined in a laboratory. Stitches may be required.