A Patch test is a patch testing procedure to diagnose allergic contact dermatitis, whereas the diagnosis of irritant contact dermatitis is based on exclusion of an allergic component. Patch test techniques for diagnosing ACD have been used for over 100 years and the present test methods are based on the established principle of using a testing material (chemical, food etc.) and a delivery device (chamber).
Patch testing attempts to reproduce the allergic reaction on the normal skin on the upper back of the patient. The patches are applied and subsequently removed after 48 hours. Reactions are read after 72 – 96 hours and often, again at 7 days after removal. The diagnostic value of patch testing depends upon the choice of test substance, the vehicle, the concentration, results interpretation and patient counseling. Patch tests are comprised of materials that occur in the home, work and/or recreational environment.
Appropriate diagnosis and management of persistent eczematous conditions such as contact dermatitis are common challenges for the dermatologist when patch testing. In 2008, the prevalence of contact dermatitis in the general U.S. population was variably estimated between 1.5% and 5.4% and reported to be the third most common reason for patients to seek consultation with a dermatologist, accounting for 9.2 million visits in 2004. It also accounts for 95% of all reported occupational skin diseases.
Contact allergen skin testing is a simple and objective scientific method available to physicians to augment the diagnostic process. Often, the patch test response is the crucial piece of information that allows for the early identification of the offending allergen(s) and confirmation of a diagnosis of Allergic Contact Dermatitis (ACD) and/or Irritant Contact Dermatitis (ICD). Once a diagnosis is achieved, the patient can then take appropriate action to avoid exposure to the allergen and, if possible, substitute with non-allergenic agents.