Laboratory Tests and Ancillaries
Patch testing
Contact Dermatitis_Diagnostics
Patch testing is the gold standard test in a patient wherein allergic contact dermatitis is suspected. This involves reproducing under patch tests of an allergic contact dermatitis in an individual sensitized to a particular antigen. This is recommended in cases where symptoms persist despite avoidance of trigger factors and topical therapy. Studies have shown that it is valuable to have the test done by a specialist in contact dermatitis with a clinic with access to an extended series of allergens for the investigation of dermatitis in specific anatomical sites, occupational groups, and chemical exposures. If the clinical diagnosis leads to atopic contact dermatitis, refer to a dermatologist to carry out a patch test and repeat open application test (ROAT) investigation. The sensitivity of patch testing is 70%, and the specificity is 80%. Specialty trays are helpful in conducting extensive patch testing for evaluation of occupational sources of dermatitis. Consider using a supplemental series of allergens depending on the patient’s exposure history.
Photopatch testing is indicated in photoallergic contact dermatitis. This is not recommended as a diagnostic test for irritant contact dermatitis.
Repeat Open Application Test (ROAT)
A repeat open application test (ROAT) is an alternative method to patch testing in detecting allergic contact dermatitis. This is a standardized exposure test mimicking an "in use" or "actual use" situation, which aims to elicit allergic contact dermatitis in the test area. This is used to clarify the clinical importance of selected positive and doubtful patch test reactions by testing cosmetics, topical drugs, and other suitable formulations. Test solutions are applied twice a day for up to 2 weeks on the volar aspect of the forearm near the antecubital fossa. A positive response in the form of eczematous dermatitis may appear after a few days in the application area. Histopathological examination of the skin biopsy may be done in atypical cases.