Skin susceptibility to dithranol: contact allergy or irritation?

A pronounced skin reaction to rather low concentrations of dithranol is often observed. The question arises as to whether this represents an allergic or irritant reaction. We performed patch tests with dithranol ointment in various concentrations on psoriatic patients and healthy volunteers. The results failed to show any clear-cut difference in skin reactions between psoriatic patients and healthy volunteers. One subject with a previously known skin hyperreactivity to dithranol showed a similar response to those of some healthy probands. However, subjects showing a pronounced response to rather low concentrations of dithranol also demonstrated a strong reaction to sodium lauryl sulphate, a standard irritant. We conclude that increased reactivity to dithranol most likely reflects an increased genuine skin susceptibility, rather than an allergic response.