How Contact Dermatitis Is Diagnosed

Your doctor can often reach a diagnosis of contact dermatitis based on your history and physical examination, but finding the exact cause might be more difficult. Contact dermatitis can be either irritant or allergic, only the latter of which can be confirmed with patch testing. If an irritant is to blame for your reaction, you'll need to work with your doctor to identify problematic substances so you can avoid them in the future.

In some cases, additional testing to rule out other concerns—including skin infection—may be performed.

Self-Checks

Many times, people are able to self-diagnose contact dermatitis and then work to avoid their triggers. Narrowing the long list of possible irritants and allergens can help pinpoint the culprit(s).

Make a list of your activities and any chemicals, household products, personal care products, cosmetics, fragrances, jewelry, and anything else that may have touched your skin in the two weeks before you had a reaction (some allergens may have a delayed presentation). For some products, the reaction may only happen after sun exposure, so be sure to note that as well.

Your job (such as hairstylist, construction worker) or hobby (such as gardening, ceramics) might help reveal the cause of your contact dermatitis. If a workplace trigger is suspected, it is important to take note of the effects of vacation, weekends, and varied work schedules on the rash.

The location of the rash on the body may be an important clue as to the cause of the contact dermatitis. However, some parts of the body are more prone to develop a rash from contact dermatitis than others.

Think about everything you come into contact with in your day.

Labs and Tests

The diagnosis of contact dermatitis should be considered when a person has any acute or chronic rash that typically itches, but may also sting or burn. The rash will appear red and may be slightly raised or bumpy.

In its severe form, the rash classically has small blisters containing clear fluid, but can swell, crust, ooze or peel in other cases.

There is no test for irritant contact dermatitis, but your doctor can test for causes of allergic contact dermatitis using a patch test. A patch test involves the placement of various chemicals on the back for approximately 48 hours (it is not the same as allergy skin prick testing). This typically is done with a pre-filled epicutaneous patch kit, such as the TRUE test.

The TRUE test is the only FDA-approved test for contact dermatitis in the United States, although some allergists and dermatologists will develop more extensive patch test panels for their patients with chemicals purchased from Canada or Europe.

The results of the test are interpreted at 48 hours after placement, and again at 72 or 96 hours after placement. A positive test is confirmed when there are blisters, redness, and/or mild swelling at the site of the particular substance in question after 72 or 96 hours. The site of the positive test usually itches, although the reaction size is typically limited to the site of contact and, therefore, is usually smaller than a dime.

It may be difficult to determine the cause of contact dermatitis since products may contain many different chemicals and an individual may use many of these different products. For example, many perfumes, lotions, creams, and other toiletries may contain various fragrances that can cause contact dermatitis.

Differential Diagnoses

Your doctor will also consider whether your rash is due to a skin infection, systemic infection, seborrheic dermatitis, atopic dermatitis (eczema), dyshidrotic eczema, psoriasis, dermatitis herpetiformis, or mycoses fungoides.

In some cases, your doctor may do a skin lesion biopsy or a culture to rule out psoriasis or infection by bacteria or fungi.

A Word From Verywell

Getting rid of an itchy rash can become a high priority. Knowing what triggers it is the first step in treatment and prevention. While your doctor will be able to test for common allergic causes, you will have to become a detective to tease out irritant causes. Bring any products you are concerned about with you to your doctor's appointment. Use the location of the rash and your list of activities and contacts so you can find out what to avoid in the future.