What is a rash evaluation?

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. They may ask you if you have started using any new soaps, lotions, cosmetics, or household products. Also, tell your provider if your skin comes in contact with any irritating substances, for example while you are at work or doing a hobby.

Your provider will also review your symptoms and examine your skin. They may also order additional tests, such as a patch test, skin culture, or skin biopsy.

A rash, also known as dermatitis, is an area of red, irritated, and usually itchy skin. 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 a threat. These substances are called allergens. When exposed to an allergen, your immune system sends out chemicals in response. These chemicals affect your skin, causing you to have an allergic reaction to the substance and 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.
• Cosmetics.
• Fragrances.
• Jewelry metals, such as nickel.
• Rubber, including latex.

Allergic contact dermatitis usually causes more itching than pain. Sometimes the rash may not develop until several days after you were exposed to the allergen. A rash may happen after the first time you are exposed to the allergen, or after you have been exposed to it repeatedly.

Irritant contact dermatitis happens when an area of your skin responds to an irritating substance. This causes a skin rash to form. Common causes of irritant contact dermatitis include:

• Household products such as detergents and drain cleaners.
• Strong soaps.
• Pesticides.
• 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. The rash usually develops quickly after exposure to the irritating substance. Irritant contact dermatitis is more common than allergic contact dermatitis.

You might be at a higher risk of developing contact dermatitis if you work around irritating chemicals or allergens, for example if you are a hairstylist, mechanic, or construction worker.

In addition to contact dermatitis, a rash may be caused by:

• Skin disorders such as eczema and psoriasis.
• Infections such as chickenpox, 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. Rashes can occur anywhere on your skin where you come into contact with an allergen or irritant. Most rashes can be treated at home with over-the-counter anti-itch creams or antihistamines. Your rash may go away quickly if you know what caused your reaction and avoid that allergen. But sometimes, a rash is a sign of a more serious condition and should be checked by a provider.

Why do I need a rash evaluation?

You may need a rash evaluation if you have symptoms that don't respond to at-home treatment. Symptoms of a contact dermatitis rash include:

• Redness or darker than natural skin tone
• Swollen or bumps that look like hives or blisters
• Itching
• Pain (more common with an irritant rash)
• Dry, cracked skin

Other types of rashes may have similar symptoms. Additional symptoms can 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 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.
• Itching that is constant.
• A rash that doesn't go away after a week of treatment.

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 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.
• You'll wear the patches for 48 to 96 hours and then return to your provider's office. You'll need to keep the patches dry.
• 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 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 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.

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 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 provider. To understand the results of a rash evaluation, your provider will consider your symptoms, medical history, and the results of other tests.

Learn more about laboratory tests, reference ranges, and understanding results.

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 baths. Other treatments will depend on your specific diagnosis.