FAQ about testing for the cause of a skin rash.

What is patch testing?

Allergic contact dermatitis occurs when your skin comes into contact with a substance that causes you to have a reaction. Patch testing is designed to identify substances that may be causing your skin rash.

During patch testing, substances to which you may be allergic are applied to your back. Your skin is then examined for allergic reactions. Patch testing does not involve scratches or pricks to the skin. It does not identify allergies to food, oral medications or inhaled substances.

How do I prepare for patch testing?

If you take cortisone pills or have had a cortisone injection within the past month, tell your health-care provider. You may use any prescribed skin creams as directed until one week before patch testing. At that time, stop applying prescribed creams to your back (you may continue to apply them to other areas).

You may continue to use moisturizers until the patch-test strips are applied. Avoid exposure to the sun or ultraviolet light (such as tanning booths) for four weeks before patch testing.

On the day you are scheduled to have your patches applied, you may wish to wear clothes you won't mind getting stained. On rare occasions, the pen used to mark the area to be tested can stain clothing.

How is patch testing done?

Patch testing involves a series of appointments, each lasting about 20 minutes. These appointments will be scheduled with the patch test nurse.

  • Day 1 - Monday
    During your first appointment, a nurse will apply several strips to your back. Each strip contains 10 dime-sized aluminum disks. Each disk contains a different substance to which you may be allergic. Additional tape may be used to hold strips in place. (Occasionally, the disks are applied to another area of the body.)
  • Day 3 - Wednesday
    After about 48 hours, you will return to have the disks removed. You will be examined for reactions such as redness, inflammation or swelling at the patch-test site. Tell the nurse if you experienced any itching or burning when the disks were in place.
  • Day 5 - Friday
    After about 96 hours, you will return for another examination. It may take that long for a skin reaction to appear.

What do my results mean?

Positive Results -- If patch testing reveals you are allergic to a substance or substances, the test is "positive." You will be given information about the substance or substances that caused your skin to react, including alternate names for them and the products in which they may be found. Examining the products you use can help you avoid substances to which you are allergic.

Remember: even if you have a positive result, it can be hard to determine exactly what product is causing your reaction and whether other factors also are involved. It may take time to lessen or eliminate your symptoms. Be patient and continue to work with your health-care provider to determine the best ways to manage your condition.

Negative Results -- If your skin shows no reaction to patch testing, your results are negative. This may occur when an allergy is not the cause of your skin problems or when the exact chemical causing the problem has not been tested. Retesting of additional chemicals is sometimes needed.

Side Effects and Risks

Although patch testing can cause a reaction at the patch-test site, the test has very few risks or side effects. You may have itching at the patch-test site after testing is completed, but this typically goes away within a few days. Contact your health-care provider promptly if a disk causes intense itching, burning or pain.