Patch Testing

Patch Testing

What is patch testing?

Patch testing is a method used to determine whether a substance in contact with the skin is causing inflammation known as contact dermatitis.

Contact dermatitis comes in two forms: irritant contact dermatitis and allergic contact dermatitis.

Irritant contact dermatitis occurs when a substance causes inflammation in almost anyone when applied in high enough concentration for a sufficient duration. This reaction doesn't involve the immune system and results from direct contact with the irritant.

Allergic contact dermatitis, however, is specific to the individual and is triggered by a substance or group of related substances called allergens. Allergy involves the immune system, and exposure to an allergen results in a rash at all skin sites in contact with it. Avoiding contact with the substance usually resolves the rash.

Patch testing aids in distinguishing between these two types of dermatitis. During the test, various substances are applied to the skin under adhesive tape for 48 hours, after which the skin is examined for any reaction. This helps identify allergens responsible for aggravating your dermatitis, allowing your doctor to recommend avoidance strategies.

Why is patch testing performed?

If you're experiencing recent-onset dermatitis or have persistent or unusual eczema, your dermatologist may suspect allergic contact dermatitis. Similarly, if using a medication in specific body areas leads to dermatitis, your doctor may suspect that it's exacerbating the condition.

Patch testing is the only definitive method to confirm if a substance is causing or worsening your dermatitis. Once identified, avoiding the allergen can lead to resolution of your dermatitis.

How is patch testing conducted?

Your doctor will first discuss your skin problem, considering factors such as the onset and progression of your rash, previous treatments, medical history, family history, cosmetic use, occupation, and hobbies. Any recent exposure to potential allergens should also be disclosed.

Your skin will then be examined for the severity and distribution of dermatitis. Although the rash is typically worst at the exposure site, it can spread if triggered by contact with other areas.

Which allergens are tested?

Your dermatologist will recommend allergens based on the European Standard Battery, covering the most common allergens responsible for 85% of allergic reactions. Additional patch tests may include allergens specific to your occupation, rash site, or personal cosmetics.

How is patch testing conducted?

Patch testing is performed on unaffected skin sites. Allergens mixed with a non-allergic base are placed on the upper back within small aluminum discs and secured with adhesive tape. After 48 hours, the patches are removed, and the reaction is assessed at one hour and 48 hours post-removal.

Interpreting the results

Reactions are scored based on severity, ranging from mild redness (doubtful reaction) to intense redness and swelling with blistering (extreme positive reaction). Distinguishing between allergic and irritant reactions is crucial; irritant reactions peak immediately after patch removal and diminish, while allergic reactions develop over days and persist.

Photo-patch testing

Some substances trigger allergic reactions only when exposed to light, warranting photo-patch testing. Identical sets of allergens are applied to the skin, with one set exposed to UVA light. A positive reaction occurs solely on the light-exposed site.

Managing positive reactions

Upon positive test results, you'll receive detailed guidance on avoiding allergens, including scrutinizing product ingredients, using barrier creams, and discussing workplace modifications with your employer.

Common allergens tested

Balsam of Peru, Caine mix, Carba mix, Chlorocresol, Chromate, Cobalt, Colophony, Epoxy resin, Formaldehyde, Fragrance mix, Lanolin, Mercapto mix/thiazoles, MBT, Neomycin, Nickel, Parabens, PPD, Primin, PTBPF resin, Thiuram, Toluene sulphonamide formaldehyde resin.