How Contact Dermatitis Is Diagnosed

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Contact dermatitis is an itchy, blistering skin rash typically caused by the direct contact of a substance with the skin. There are two types of contact dermatitis: irritant and allergic. This difference is often difficult to tell apart and is not usually an important distinction to make, but your doctor can only test for allergic causes. You will need to work with your doctor to identify irritant sources so you can avoid them in the future.

Self Checks/At-Home Testing

Many times people self-diagnose contact dermatitis and then are able to avoid the triggers. Narrowing the long list of possible irritants and allergens can help pinpoint the culprit. 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 the reaction. For some products, the reaction may only happen after sun exposure, so be sure to note that as well.

The location of the rash on the body may give important clues as to the cause of the contact dermatitis. However, some parts of the body are more prone to develop a rash from contact dermatitis.

Eyelids. Contact dermatitis of the eyelids is extremely common, especially because the thin skin of the eyelids is prone to developing a rash. Aside from the obvious locally applied cosmetics that can cause contact dermatitis of the eyelids, nail polishes are a frequent cause from touching the eyes.

Other causes of eyelid contact dermatitis include:

  • Airborne triggers (such as pollens and fumes)
  • Facial cosmetics
  • Hair dye
  • Nail polishes
  • Artificial nails
  • Fingernail coatings and resins
  • Eyelash curlers
  • Mascara and mascara applicators
  • Facial tissues with perfumes or lotions added
  • Shampoos and conditioners
  • Hair gel, mousse, and sprays

Face. Cosmetics and fragrances are the most common causes of facial contact dermatitis, although the face can be affected by substances on the hands or other parts of the body. Substances applied to the scalp (such as hair dye, shampoos, and hairspray) may affect the face, even if the scalp is not affected.

Other causes of facial contact dermatitis include:

  • Many of the causes of eyelid dermatitis
  • Sunblocks
  • Topical medications
  • Facial masks
  • Rubber sponges
  • Children’s toys
  • Balloons
  • A partner’s facial cosmetics or fragrances

Scalp. The skin on the scalp is fairly resistant to contact dermatitis, and may not be affected even if hair dye is the cause of eyelid or facial contact dermatitis. Hair dye is the most common trigger of scalp contact dermatitis. It is not uncommon for a person to develop a problem to a hair dye even if the same product has been used for years.

Other triggers can include anything applied to the scalp such as shampoo, conditioners, hair gels, mousse, straighteners, and relaxers.

Hands. Hand contact dermatitis is extremely common and usually affects the thinner skin on the back of the hand rather than the thicker skin of the palms. Occupational causes of hand dermatitis are common, especially “wet” occupations such as food processors, housekeepers, and health professionals.

Neck. The skin on the neck is very reactive and susceptible to contact dermatitis from various sources:

  • Many of the same causes of scalp and facial contact dermatitis
  • Perfumes
  • Jewelry

Underarms. Contact dermatitis to the underarms can be due to topically applied agents as well as chemicals leaching out of clothing. Antiperspirants, with and without deodorants, can cause dermatitis of the underarms. Chemicals that leach out of clothing as a result of sweating, such as dyes, resins, and formaldehyde are other causes. Shaving and depilatory agents for removal of hair are other common causes of underarm contact dermatitis.

Legs. Common causes of contact dermatitis of the legs include topical medications, shaving agents, moisturizers, and stocking materials and dyes. Brushing up against poison oak and poison ivy plants can lead to contact dermatitis in a classic linear pattern.

Genital/rectal area. Common causes of contact dermatitis of the genital/rectal area include topical medications, suppositories, douches, latex condoms, diaphragms, lubricants, spermicides, sprays and fragrances from toilet paper, soaps, bubble bath, and diapers. It is important to consider agents transferred from the hands, including nail polish and chemicals from plants, including poison oak and ivy.

Other causes of dermatitis in this area include:

  • Ammonia in the urine in infants and adults with incontinence
  • Anal itching can be worsened by ingesting spices, antibiotics, and laxatives

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 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. The 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 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 chemical 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.

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

Differential Diagnoses

Your doctor will consider whether your rash is due to a skin infection or a systemic infection. These are often easy to distinguish from contact dermatitis, but it can be complicated if you have been scratching the area and have developed a skin infection in addition to contact dermatitis. The diagnosis will then be aimed at finding what is triggering your contact dermatitis.

It may be difficult to determine the cause of contact dermatitis since some common chemicals that cause contact dermatitis are found in many substances. For example, many perfumes, lotions, creams, and other toiletries may contain various fragrances which cause contact dermatitis.

A reaction can develop to a cosmetic, hair dye, toiletry or any other trigger even if the substance has been used for years without problems.

Your job or hobby may give clues to the cause of contact dermatitis. If a workplace trigger is suspected, it is important to know the effects of vacation, weekends, and varied work schedules on the rash. Contact with chemicals in a hobby, such as painting, ceramics, photography, and gardening may be important clues.

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 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.

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