Nonanoic acid irritation - a positive control at routine patch testing?

A false negative patch test can be related to errors in the testing procedure, or to a defect in the eliciting (efferent) phase of the immune response. To define this phase the application of a topical irritant (a "positive control") is suggested. Various concentrations of nonanoic acid in 1-propanol were applied to 116 healthy volunteers and to 75 dermatitis patients. Dose-response curves were obtained (concentrations 5, 10, 20 and 39.9%). Twenty percent nonanoic acid produced skin reactions in 90.1--93.9% of the subjects. The lesions consisted mainly of erythema at 48 h and pigmentation at 96 h. We suggested that the most interesting patients are those negative to this primary irritant and thay they should be further evaluated.