A rash evaluation is a test to find out what is causing a rash. A rash, also known as dermatitis, is an area of skin that is red, irritated, and usually itchy. A skin rash may also be dry, scaly, and/or painful. Most rashes happen when your skin touches a substance that irritates it. This is known as contact dermatitis. There are two main types of contact dermatitis: allergic contact dermatitis and irritant contact dermatitis.
Allergic contact dermatitis happens when your body’s immune system treats a normally harmless substance as if it were a threat. When exposed to the substance, the immune system sends out chemicals in response. These chemicals affect your skin, causing you to develop a rash. Common causes of allergic contact dermatitis include:
- Poison ivy and related plants, like poison sumac and poison oak. A poison ivy rash is one of the most common types of contact dermatitis.
- Jewelry metals, such as nickel.
Allergic contact dermatitis usually causes itching that can be severe.
Irritant contact dermatitis happens when a chemical substance damages an area of skin. This causes a skin rash to form. Common causes of irritant contact dermatitis include:
- Household products such as detergents and drain cleaners
- Strong soaps
- Nail polish remover
- Body fluids, such as urine and saliva. These rashes, which include diaper rash, most commonly affect babies.
Irritant contact dermatitis is usually more painful than itchy.
In addition to contact dermatitis, a rash may be caused by:
- Skin disorders, such as eczema and psoriasis
- Infections such as chicken pox, shingles, and measles
- Insect bites
- Heat. If you get overheated, your sweat glands can get blocked. This can cause a heat rash. Heat rashes often happen in hot, humid weather. While it can affect people of any age, heat rashes are most common in babies and young children.
Other names: patch test, skin biopsy
What is it used for?
A rash evaluation is used to diagnose the cause of a rash. Most rashes can be treated at home with over-the-counter anti-itch creams or antihistamines. But sometimes a rash is a sign of a more serious condition and should be checked by a health care provider.
Why do I need a rash evaluation?
You may need a rash evaluation if you have rash symptoms that aren’t responding to at-home treatment. Symptoms of a contact dermatitis rash include:
- Pain (more common with an irritant rash)
- Dry, cracked skin
Other types of rashes may have similar symptoms. Additional symptoms vary depending on the cause of the rash.
While most rashes are not serious, in some cases a rash can be a sign of a serious health condition. Call your health care provider if you or your child has a skin rash with any of the following symptoms:
- Severe pain
- Blisters, especially if they affect the skin around the eyes, mouth, or genitals
- Yellow or green fluid, warmth, and/or red streaks in the rash area. These are signs of infection.
- Fever. This could be a sign of a viral or bacterial infection. These include scarlet fever, shingles, and measles.
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
What happens during a rash evaluation?
There are different ways to do a rash evaluation. The type of test you get will depend on your symptoms and medical history.
To test for allergic contact dermatitis, your health care provider may give you a patch test:
During a patch test:
- A provider will place small patches on your skin. The patches look like adhesive bandages. They contain small amounts of specific allergens (substances that cause an allergic reaction).
- You’ll wear the patches for 48 to 96 hours and then return to your provider’s office.
- Your provider will remove the patches and check for rashes or other reactions.
There is no test for irritant contact dermatitis. But your provider may make a diagnosis based on a physical exam, your symptoms, and information you provide about your exposure to certain substances.
A rash evaluation may also include a blood test and/or a skin biopsy.
During a blood test:
A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out.
During a biopsy:
A provider will use a special tool or a blade to remove a small piece of skin for testing.
Will I need to do anything to prepare for the test?
You may need to stop taking certain medicines before the test. These include antihistamines and antidepressants. Your health care provider will let you know which medicines to avoid and how long you need to avoid them before your test.
Are there any risks to the test?
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 health care provider.
There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.
After a biopsy, you may have a little bruising, bleeding, or soreness at the biopsy site. If these symptoms last longer than a few days or they get worse, talk to your provider.
What do the results mean?
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.
If you had a blood test, abnormal results may mean you:
- Are allergic to a certain substance
- Have a viral, bacterial, or fungal infection
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
If you have questions about your results, talk to your health care provider.
Is there anything else I need to know about a rash evaluation?
To relieve symptoms of a skin rash, your provider may suggest over-the-counter medicines and/or at-home treatments, such as cool compresses and cool baths. Other treatments will depend on your specific diagnosis.
Courtesy of MedlinePlus from the National Library of Medicine.