Contact dermatitis and patch testing

Irritant versus allergic

Irritant contact dermatitis is the more common type. Repeated exposure to water, soaps, detergents, solvents, friction or cold wears down the skin barrier until it becomes dry, cracked, red and sore. It can affect anyone given enough exposure, which is why it is common in health care workers, cleaners, hairdressers, food handlers, mechanics and parents of young children. The hands are affected most often.

Allergic contact dermatitis occurs only in people who have become sensitised to a specific substance. Once sensitised, even a small amount causes an itchy, red, sometimes blistering rash one to three days after contact, at the site of contact and sometimes beyond it. The allergy is usually lifelong, so identifying the substance and avoiding it is essential.

Both types often occur together, and both can complicate pre existing atopic dermatitis.

Common allergens

• Nickel, in jewellery, belt buckles, zips, coins, phones and keys
• Fragrance ingredients, in perfumes, cosmetics, soaps and household products, including many labelled unscented
• Preservatives, in cosmetics, wet wipes, shampoos, paints and liquid soaps
• Rubber chemicals, in gloves, footwear and elastic
• Hair dye ingredients
• Topical antibiotics and other medicated creams applied to broken skin
• Acrylates, in gel and acrylic nails, adhesives and some medical devices
• Chromium in leather and cement, cobalt, and plant allergens such as poison ivy

Assessment at DermCare

History is the most important part of the assessment. Your dermatologist asks about your work, hobbies, skin care and cosmetic products, gloves and protective equipment, jewellery, and everything applied to the affected skin, along with whether the rash improves when you are away from work or on holiday. The distribution of the rash often points to the cause: the eyelids and face suggest airborne or transferred allergens such as nail products, the wrists and neck suggest jewellery, the feet suggest footwear. Bring the products you use, or images of their ingredient labels.

Patch testing explained

Patch testing is the standard investigation for suspected allergic contact dermatitis. Small amounts of common allergens, and where indicated the patient's own products, are placed in chambers taped to the upper back. The patches stay in place for 48 hours, during which the back must be kept dry and free from heavy exertion. They are then removed and the skin is read, and read again around day four or five, and sometimes day seven, because reactions to some allergens appear late. A positive reaction is a small patch of eczema at the test site. Your dermatologist then judges whether each positive result explains your rash, and provides written advice on where the substance is found and how to avoid it.

Patch testing does not test for food allergy or for immediate reactions such as hives; those are investigated by allergists using different methods.

Treatment options

Avoidance and protection

For irritant dermatitis: reduce wet work, use lukewarm water and a soap substitute, dry the hands thoroughly, apply a plain moisturiser after every wash, and wear cotton liners under protective gloves. For allergic dermatitis: avoid the identified allergen completely, using the written information provided to check labels and find alternatives.

Treating the inflammation

Topical corticosteroids of appropriate strength settle active dermatitis. Topical calcineurin inhibitors are used for the face and for maintenance. Severe or widespread reactions may need a short course of oral corticosteroids. Chronic hand eczema that persists despite avoidance and topical therapy may be treated with phototherapy or systemic therapy.

For referring physicians

Please include the distribution and duration of the rash, the patient's occupation and relevant exposures, products used, whether the rash clears away from work, and treatments tried. Note any suspected occupational cause, since documentation may be needed for a workplace claim. How to refer.

Frequently asked questions