Patch Testing

Patch testing is an essential diagnostic tool used to determine whether a patient's dermatitis is caused or exacerbated by contact with a specific allergen. It is important to distinguish patch testing from skin prick testing, as the two serve different clinical purposes. Skin prick tests have limited value in patients with chronic or recurrent skin rashes and are primarily used to diagnose immediate-type allergies such as hay fever related to grass pollens, house dust mites, or animal dander. In contrast, patch testing is specifically designed to identify delayed hypersensitivity reactions responsible for allergic contact dermatitis.

A wide variety of substances can be used during patch testing. Most dermatology clinics utilise a standardised panel of allergens, commonly referred to as the European Standard Series, which includes substances such as nickel, parabens, fragrance mixes, preservatives, and rubber chemicals. These allergens are applied to the patient's upper back in carefully measured concentrations to provoke an allergic reaction in sensitised individuals while minimising the risk of irritation in those who are not allergic.

Patch test results are not always straightforward and can occasionally be inconclusive. In some cases, patients may develop widespread redness and itching across many test sites rather than the expected one or two localised reactions. This phenomenon, known as an "angry back," is typically associated with active or severe underlying dermatitis. False-negative results can also occur, where a substance known to cause dermatitis in daily life does not elicit a visible reaction during testing. For these reasons, additional testing or extended allergen panels may be required in certain patients to fully clarify the cause of their dermatitis.

Each patch testing appointment typically lasts approximately 30 minutes. During the first visit, small amounts of up to 80 standardised allergens are applied to the upper back using individual plastic or aluminium chambers. These are secured with hypoallergenic adhesive tape to ensure stability. The patches must remain undisturbed for 48 hours. 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. At the third appointment, the dermatologist examines the back for signs of allergic reactions, ensuring that the markings remain visible for accurate interpretation.

A detailed report is completed following the 48-hour and 96-hour readings. Each test site is graded according to standardised criteria, ranging from negative results to extreme positive reactions. Irritant reactions may appear as sweat rashes, follicular pustules, or burn-like changes, while 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.

During the testing period, patients are advised to avoid activities that could dislodge the patches, including swimming, vigorous exercise, or 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 occasionally trigger flares of dermatitis at other body sites, requiring treatment with prescribed topical medications.