Allergic Contact Dermatitis: Common Allergens and Patch Testing

6 days ago

3 min read

Allergic contact dermatitis (ACD) is a skin rash caused by a delayed (Type IV) allergic reaction when the skin touches a substance it has previously become allergic to. The rash usually appears within 24-72 hours after contact, at the spot that touched the substance. In the acute phase it is red, itchy and blistering; in the chronic phase the skin becomes thick and cracked. The key to treatment is finding the allergen through a skin allergy test (patch test). This article explains common allergens, testing, and treatment by a dermatologist.

What causes allergic contact dermatitis?

ACD results from a T-cell-mediated immune response to an allergen. On first contact, the body builds an allergic memory (sensitization), and the rash appears on later re-exposure. This differs from irritant contact dermatitis, which can occur on the very first contact without any prior allergy.

Common allergens

Nickel: The most common allergen, found in earrings, necklaces, belt buckles and trouser zippers. The rash appears clearly where the metal touches the skin.
Fragrance: From perfumes and cosmetics.
Preservatives: Such as methylisothiazolinone and formaldehyde-releasing agents in cosmetics and shampoos.
Rubber chemicals: From rubber gloves, often causing hand rashes, commonly seen in healthcare workers.
PPD (hair dye): Found in dark hair dyes, and can cause a severe, swollen rash on the scalp.
Topical medicines: Some topical antibiotics, such as neomycin.

Occupational contact dermatitis (hand dermatitis)

Hand rashes in people with certain jobs are often a mix of allergic and irritant contact dermatitis. Assessing the occupation together with patch test results helps identify the true trigger. Commonly affected groups include healthcare workers, hairdressers, housekeepers, workers exposed to chemicals, and people who wash their hands or touch water frequently.

Skin allergy test (patch test)

The patch test is the standard method for confirming allergens. The doctor applies test patches containing several standard allergens to your back.

Day 1: The test patches are applied to your back.
About 48 hours: The patches are removed and the first reading is taken.
About day 4: A second reading is taken to confirm, because some reactions appear late.

Before a patch test, you should stop oral steroids and topical medicines on the back as your doctor advises, so they do not mask the reading.

Treatment

Avoid the allergen: Avoiding the allergens that were found is the most important part of treatment.
Topical steroids: Topical steroids help reduce inflammation in the acute phase, and the doctor chooses a strength suited to the site and severity.
Other medicines: Tacrolimus or pimecrolimus are options for thin skin such as the eyelids, to reduce steroid side effects.
Skin care and itch relief: Apply moisturizer often and use antihistamines to relieve itching as your doctor advises.
Hand protection: People with hand rashes should wear suitable protective gloves when handling triggers.

Frequently asked questions

How is allergic contact dermatitis different from irritant contact dermatitis? Allergic contact dermatitis (ACD) requires a previous allergy to the substance, while irritant contact dermatitis (ICD) can occur from the first contact with a skin-irritating substance. A patch test helps confirm that it is ACD.

Does a patch test hurt? No. It is only a set of test patches placed on the back, but some people may feel itching at the spot where they are allergic.

How should I prepare for a patch test? You should stop oral steroids and topical medicines on the back as your doctor advises, and avoid getting your back wet or sweating heavily from exercise while the patches are on.

Can allergic contact dermatitis be cured? If you can avoid the allergen, the rash usually clears and does not return, but continued exposure to the substance may turn it into a chronic rash.

Finding the allergen is the key to treating allergic contact dermatitis for the long term. If you have a chronic itchy rash that you suspect is contact allergy, consult a dermatologist and ask about a patch test at Siam Dermatology. Call +66 61 448 7000 or LINE @siamderm.

Written by Dr. Patara Voravutinon, Dermatologist · Last reviewed July 16, 2026