ABSTRACT
Dithranol is a natural anthraquinone derivative with anti-psoriatic and anti-inflammatory properties. It controls skin growth by reducing DNA synthesis and mitotic activity in the hyperplastic epidermis, restoring a normal rate of cell proliferation and keratinization. The irritant properties of dithranol are well known, and it is likely that a number of cases considered to be allergic on the basis of a positive patch test reaction, in fact had shown a wrongly interpreted false-positive, irritant patch test reaction, especially with test concentrations of 0.005% or higher. Conversely, as all physicians are well aware of irritation and irritant dermatitis from dithranol, cases of real allergic contact dermatitis may be interpreted as irritant; consequently, no patch tests are performed and sensitization goes unnoticed. Co-reactivity with coal tar has been observed several times. Possible explanations are independent sensitization in patients having used both coal tar and dithranol, a shared allergen, or the presence of minute quantities of dithranol in coal tar.
