Rash Evaluation
Reviewed
Also called: Patch test, Skin biopsy
A rash evaluation is a test to find out what is causing a rash. As part of the evaluation, your health care provider will ask about your medical history and your rash.
Specimen: skin (patch test), skin tissue (biopsy), blood (blood test)
Urgent safety information
Sometimes, a rash can be the first sign of a severe and dangerous allergic reaction called anaphylaxis. Call 911 or seek immediate medical attention if the rash is sudden and spreads quickly, you have trouble breathing, your face is swollen.
Emergency numbers on this page are for the United States. Outside the US, call your local emergency number.
Why this test may be ordered
- A rash that doesn't respond to at-home treatment
- To find out if a rash is caused by an allergen or irritant
- Signs of infection or fever with a rash
Preparation and collection
You may need to stop taking certain medicines before this test, so tell your provider about everything you take. Some medicines may affect the results of your test. But don't stop any medicines unless your provider tells you to.
There are different ways to do a rash evaluation. The type of test you get will depend on your symptoms and medical history.
There is very little risk to having a patch test. If you feel intense itching or pain under the patches once you are home, remove the patches and call your provider.
Understanding results
To understand the results of a rash evaluation, your provider will consider your symptoms, medical history, and the results of other tests.
- Positive patch test: If you had a patch test and have itchy, red bumps or swelling at any of the testing sites, it means you are probably allergic to the substance tested. Limitation: Sometimes the rash may not develop until several days after you were exposed to the allergen. Next step: Your rash may go away quickly if you know what caused your reaction and avoid that allergen.
- Abnormal blood test: If you had a blood test, abnormal results may mean you are allergic to a certain substance, have a viral, bacterial, or fungal infection. Limitation: To understand the results of a rash evaluation, your provider will consider your symptoms, medical history, and the results of other tests. Next step: If you have questions about your results, talk to your provider.
- Abnormal skin biopsy: If you had a skin biopsy, abnormal results may mean you have a skin disorder such as psoriasis or eczema, have a bacterial or fungal infection. Limitation: Other treatments will depend on your specific diagnosis. Next step: If you have questions about your results, talk to your provider.
- There is no test for irritant contact dermatitis.
- Patch test reactions may take days to appear.
What to do next
If you have questions about your results, talk to your provider.
- What do you think is causing my rash?
- Which tests do I need?
- What should I avoid while my rash heals?
Evidence source
Source: U.S. National Library of Medicine (MedlinePlus). Practices and reference ranges may differ in your country.