A comparison of the dose-response relationship for psoralen-UVA erythema and UVB erythema

Twenty patients with psoriasis were phototested to determine their erythemal responses to UVB and psoralen-UVA (PUVA) (oral 8-methoxypsoralen). The smallest ultraviolet radiation doses to produce erythema (minimal erythema dose and minimal phototoxic dose, respectively) were recorded and dose-response curves were constructed for UVB (24 hours after irradiation) and PUVA (48 hours) using objective measurement. The choice of a 48-hour measurement was validated by phototesting an additional 11 subjects to determine the time course of PUVA erythema. No correlation was demonstrated between minimal erythema dose for UVB, minimal phototoxic dose for PUVA, and sun-reactive skin type. The mean slope of the dose-response curve for UVB erythema was four times steeper than that for PUVA. Psoralen-UVA erythema reached a broad maximum between 48 and 96 hours after irradiation. Using objective methods we have demonstrated that the commonly accepted view of a steep dose-response relationship for PUVA erythema is not valid.