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.

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 days before you had a reaction. For some products, the reaction may only happen after sun exposure, so be sure to note that as well.

Your job (e.g., hairstylist, construction worker) or hobby (e.g., 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.

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 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 testing). This typically is done with a paper tape system, 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 mild swelling at the site of the particular substance in question. 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 some common chemicals that cause the condition are found in many products. For example, many perfumes, lotions, creams, and other toiletries may contain various fragrances that can cause contact dermatitis.

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. These are often easy to distinguish by your history and your doctor's physical examination, but this can be complicated if you have been scratching the area and have developed a skin infection in addition to contact dermatitis.

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

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 allergic causes, you will have to become a detective to tease out irritant causes. Use the location of the rash and your list of activities and contacts so you can find out what to avoid in the future.