Patch Testing

Patch testing is an essential diagnostic procedure used to determine whether dermatitis is caused or worsened by contact with a specific allergen. It is the standard method for identifying delayed hypersensitivity reactions responsible for allergic contact dermatitis. Patch testing should be distinguished from skin prick testing, which is used to diagnose immediate-type allergies such as those related to grass pollens, house dust mites, or animal dander. Skin prick tests have limited value in patients with chronic or recurrent skin rashes.

Allergen Panels

A wide variety of substances may be used during patch testing. Most dermatology clinics utilize a standardized panel of allergens, commonly referred to as the European Standard Series. This panel includes substances such as nickel, parabens, fragrance mixes, preservatives, and rubber chemicals. These allergens are applied in carefully measured concentrations designed to provoke a reaction in sensitized individuals while minimizing the risk of irritation in those who are not allergic.

The Testing Procedure

Patch testing is conducted over three appointments, each lasting approximately 30 minutes.

First Appointment: Application

During the first visit, small amounts of up to 80 standardized allergens are applied to the upper back using individual plastic or aluminium chambers. These chambers are secured with hypoallergenic adhesive tape to ensure stability. The patches must remain undisturbed for 48 hours.

Second Appointment: Patch Removal

At the second appointment, the patches are removed and the test sites are carefully marked with an indelible marker to allow accurate identification during subsequent readings.

Third Appointment: Final Reading

At the third appointment, the dermatologist examines the back for signs of allergic reactions. The markings from the previous visit are used to ensure accurate interpretation of each test site.

Interpreting Results

A detailed report is completed following the 48-hour and 96-hour readings. Each test site is graded according to standardized criteria. Irritant reactions may appear as sweat rashes, follicular pustules, or burn-like changes. Uncertain reactions present as faint pink discolouration. Weak positive reactions appear as slightly raised red plaques, strong positive reactions show papulovesicles, and extreme reactions may include blistering or ulceration.

The clinical relevance of each positive reaction is determined by correlating test results with the patient's exposure history, pattern of dermatitis, and known allergen sources. Patch test results are not always straightforward and can occasionally be inconclusive. In some cases, widespread redness and itching may develop across many test sites rather than the expected one or two localized reactions. This phenomenon, known as an "angry back," is typically associated with active or severe underlying dermatitis. False-negative results can also occur. For these reasons, additional testing or extended allergen panels may be required to fully clarify the cause of dermatitis.

Patient Instructions During Testing

During the testing period, activities that could dislodge the patches must be avoided, including swimming, vigorous exercise, and excessive sweating. The back must be kept dry, and no topical products such as lotions, oils, or steroid creams should be applied to the test area until testing is complete.

Positive patch test reactions represent small areas of active eczema and may be itchy or uncomfortable. These reactions can persist for several weeks and may occasionally trigger flares of dermatitis at other body sites, which can be managed with prescribed topical medications. At the Centre for Medical and Surgical Dermatology, Dr. Maksym Breslavets provides comprehensive evaluation, expert interpretation of patch test results, detailed allergen avoidance counselling, and tailored treatment plans to control active dermatitis and prevent future flare-ups.