Provocative light testing was performed in ten patients with polymorphous light eruption (Pmle) and in ten normal subjects to determine whether the histopathology of irradiated test sites could be used as an end point for a positive response. Three separate test sites on the lower back were irradiated (UV-B) with a single exposure to i minimal erythema dose (MED), a single exposure to 5 x MED, or three consecutive daily exposures to 3 x MED. Biopsies of all test sites were coded and examined without knowing if the biopsy was from a normal subject or a patient with Pmle.
In no case was a rash reproduced by light testing. A spectrum of histologic features were observed at the three test sites in both normal subjects and patients with Pmle with no significant differences noted between the two groups. It was concluded that the histology at these UV-B irradiated test sites could not be used as an endpoint Pmle for testing. In establishing Pmle light testing techniques which rely on histology it is mandatory to include adequate numbers of normal controls.