Air-borne contact dermatitis can be diagnosed using which of the following investigation?

The most widely available method for confirming the diagnosis of air borne contact dermatitis(ACD) is patch testing. Patch test documents the presence of delayed type hypersensitivity, the results of which are read at 48 and 96 hours after placement of the allergen. Patch tests are especially indicated in cases characterized by recurrent episodes of ACD, in cases in which there is a need to identify the offending allergen, and in cases recalcitrant to conventional therapy. Positive patch test does not definitively identify the cause of ACD. False positives may be due to an irritant effect. In these cases, patch testing in control individuals can be of great value. Ref: Clinical Immunology,Principles and Practice (Expe Consult - Online and ... edited by Robe R. Rich, page 541.