Patch testing

It is a technique used to diagnose contact allergic dermatitis based on the principle of delayed hypersensitivity (an immune response). Evidence of contact allergy is derived from a patient history (such as occupation), clinical examination and patch testing. The aim of the patch test is to ascertain allergic contact dermatitis by aiming to reproduce a rash on a small controlled area of skin using standardised batches or trays of allergens (termed batteries) or those commonly used at work or home. Standard batteries of substances (now often pre-prepared) are comprised of patches made up of Finn chambers and hypoallergenic tape that are applied to the patient’s upper back; they should incorporate probable (standard battery) and possible substances (e.g. occupational specific) based on their history. The results are read at two stages, 48 hours and 72 hours; this timing sequence is related to the type IV hypersensitivity reaction, which is a delayed immune response. Care is needed to avoid misleading results from contact irritants that are distinct from hypersensitivity reactions. Differentiation is not always easy, however, the use of standard allergens and rigorous technique are required. A key reference source is a content on contact and occupational dermatology by Marks et al. (2002). Further details and sources are given in Eczema.