rash_evaluation
Reviewed
Also called: patch test, skin biopsy
A rash evaluation is a procedure to find out what is causing a rash. It includes a medical history, physical exam, and may involve additional tests such as a patch test, skin culture, or skin biopsy. Most rashes are due to contact dermatitis (allergic or irritant), but other causes include skin disorders, infections, insect bites, or heat.
Specimen: skin (patch test), skin tissue (biopsy), blood (blood test)
Urgent safety information
Call 911 or seek immediate medical attention if the rash is sudden and spreads quickly, you have trouble breathing, or your face is swollen.
Why this test may be ordered
- To diagnose the cause of a rash that does not respond to at-home treatment
- To identify allergens causing allergic contact dermatitis
- To evaluate symptoms such as redness, swelling, blisters, itching, pain, or dry cracked skin
- To rule out serious conditions when rash is accompanied by severe pain, blisters in sensitive areas, signs of infection, fever, or constant itching
Preparation and collection
You may need to stop taking certain medicines before this test. Tell your provider about everything you take. Do not stop any medicines unless your provider tells you to.
For a patch test: small patches containing allergens are placed on your skin and worn for 48 to 96 hours, then removed by your provider. For a blood test: a blood sample is taken from a vein in your arm. For a biopsy: a small piece of skin is removed using a special tool or blade.
Patch test: very little risk; if you feel intense itching or pain under the patches, remove them and call your provider. Blood test: very little risk; slight pain or bruising at the needle site, usually goes away quickly. Biopsy: slight bruising, bleeding, or soreness at the site; if symptoms last longer than a few days or worsen, talk to your provider.
Understanding results
Your provider will consider your symptoms, medical history, and the results of other tests to understand the results of a rash evaluation.
- Patch test: itchy, red bumps or swelling at any testing site: You are probably allergic to the substance tested. Limitation: This is a diagnostic test for allergic contact dermatitis. False positives or negatives are possible; interpretation should be done by your provider. Next step: Prompt
- Patch test: no reaction at testing sites: No allergy detected to the tested substances. Limitation: Does not rule out other causes of rash. Some reactions may be delayed. Next step: Routine
- Blood test: abnormal results: You may be allergic to a certain substance, or have a viral, bacterial, or fungal infection. Limitation: Results are not diagnostic alone; your provider will correlate with symptoms and other tests. Next step: Prompt
- Blood test: normal results: No significant abnormalities detected in the blood sample. Limitation: Does not rule out all causes of rash. Next step: Routine
- There is no test for irritant contact dermatitis; diagnosis is based on physical exam, symptoms, and exposure history.
- Results from patch tests, blood tests, and biopsies must be interpreted by your provider in the context of your overall health.
- A rash may be a sign of a serious condition; seek emergency care if symptoms like trouble breathing or rapid spreading occur.
What to do next
To relieve symptoms, your provider may suggest over-the-counter medicines and/or at-home treatments such as cool compresses and baths. Other treatments depend on your specific diagnosis.
- What is the most likely cause of my rash based on the evaluation?
- Do I need any follow-up tests or treatments?
- Are there any substances I should avoid to prevent future rashes?