Allergic contact dermatitis: causes, patch testing and treatment

The NHS describes contact dermatitis as a type of eczema triggered by contact with a particular substance. There are two main forms. Irritant contact dermatitis happens when a substance directly damages the outer layer of the skin, and it is the more common form. Allergic contact dermatitis is less common and happens when the immune system responds to an allergen in a way that affects the skin.

NICE CKS describes the allergic form as a delayed hypersensitivity reaction that occurs after sensitisation and then re-exposure to a specific allergen. In practice this means that you can use a product for months or years without trouble, become sensitised, and then react each time you touch it again. The BAD adds that it is unclear why some people become allergic while others do not, and that allergic contact dermatitis is not linked to having eczema in childhood.

The BAD says contact dermatitis is common and can have a long-term impact on daily activities, emotional wellbeing and social interactions.

The BAD lists personal care products including cosmetics, skincare products, preservatives, nail varnish, fragrances, sunscreen and hair dye. It also names metals in jewellery such as nickel and cobalt, and chromate, a metal found in cement. NICE CKS adds topical medicines, including some antibiotics and corticosteroids, and plants such as sunflower and primula. The NHS says handling raw, uncooked fruit and vegetables may also trigger a reaction in some people.

The BAD describes a related problem called protein contact dermatitis, in which certain proteins in raw foods such as uncooked potatoes cause itching and swelling known as contact urticaria, mostly in food handlers.

The NHS says contact dermatitis makes skin itchy, blistered, dry and cracked, and usually appears within a few hours or days of exposure. Lighter skin can become red, while darker skin can become dark brown, purple or grey. It most commonly affects the hands and face.

The BAD says allergic contact dermatitis usually starts as an itchy rash that looks red, inflamed and blistered, and the skin can peel and ooze. Irritant dermatitis is more often described as burning, stinging and soreness. With repeated exposure, the BAD says skin can become thickened and leathery with painful splits. Hand involvement can reduce hand function.

The distribution can hint at the trigger. NICE CKS says the anatomical distribution may aid diagnosis, though it notes that allergic, irritant and atopic dermatitis are not easily distinguished by looking alone.

A GP will ask about the rash, your job, hobbies, products and jewellery. The NHS says a GP can try to identify the cause and suggest treatment, and may refer you to a dermatologist if the substance cannot be identified or if symptoms are not responding to treatment.

The BAD says patch testing is needed to find the exact cause of allergic contact dermatitis. It is carried out in a specialist dermatology department, with patches containing common allergens placed on the back. They are removed after 2 days and the skin examined at that point and again after a further 2 days. A small rash at a site is called a positive patch test, after which the dermatologist advises which allergens to avoid. The BAD says patch testing is usually done when the cause is unknown or the rash does not respond to creams and ointments.

The NHS says that if you can avoid the trigger the skin will eventually clear, and that when avoidance is not possible, emollients and topical corticosteroids may be advised. The BAD says it may take several months for contact dermatitis to settle, and that moisturisers and soap substitutes help restore the skin barrier. It suggests thicker emollients for the hands in the evenings and lighter or gel-based ones frequently through the day, and cool compresses or oatmeal baths for sudden flare-ups.

If moisturisers are not enough, the BAD says a healthcare professional may recommend a topical steroid, and topical calcineurin inhibitors such as tacrolimus or pimecrolimus are a steroid-free option. It stresses using the right strength for the right length of time. For severe, recurrent or widespread disease the GP or nurse can refer to a dermatologist, and treatments by mouth may be needed, including steroid tablets and antibiotics for infection and, rarely, medicines such as ciclosporin or methotrexate. The NHS notes that oral corticosteroids are rarely needed.

The NHS recommends rinsing skin with warm water and an emollient as soon as possible after contact with a known allergen or irritant, using gloves to protect the hands but taking them off every now and again because sweating can worsen symptoms, wearing cotton gloves under rubber gloves if rubber also irritates, checking the ingredients of make-up and soap, and applying emollients frequently and in large amounts, with emollient soap substitutes instead of regular soap.

The BAD says continued contact with a triggering substance will make the condition worse, and that in some cases it may mean changing jobs, for example a hairdresser allergic to hair dye, a nail technician allergic to acrylates or a builder allergic to cement. Speak to your employer or occupational health team about safer products or protective measures before a change of job is considered.

See a pharmacist if symptoms are troubling you: the NHS says they can recommend emollients. See a GP for persistent, recurrent or severe symptoms. Seek prompt medical advice if the rash looks infected, for example with yellow crusts, weeping, increasing pain or a fever, because the BAD notes that bacterial infection can lead to impetigo, which is very contagious. If the rash is spreading rapidly, affects the eyes or you feel generally unwell, contact NHS 111. If you have swelling of the lips or tongue, difficulty breathing or feel faint after contact with something, call 999, as that points to a different, immediate kind of allergic reaction.

On the NHS, a GP can refer to a dermatology department, which can perform patch testing, and some occupational health services arrange it for work-related cases. Waiting times for NHS dermatology vary between areas.

Private dermatologists and some allergy clinics also offer patch testing, sometimes sooner. It is worth asking which allergen series is used, whether results include advice on which everyday products are safe, and whether findings will be sent to your GP. Patch testing should be supervised by a clinician, because the readings after two days and again later need to be interpreted in context.