At various times of the year the protoporphyrin level in the plasma of 11 patients with erythropoietic protoporphyria was determined. In the winter as well as in the summer, when the patients received carotenoids, an area on one forearm of 10 patients was irradiated with artificial (xenon arc) light, after which suction blisters were made on both forearms. Protoporphyrin levels were determined in the plasma and in the blister fluid from both areas. Suction-blister fluid was considered representative of the interstitial fluid in the dermis. In the winter the protoporphyrin levels of the blister fluid from the irradiated area were significantly higher than in the fluid from the non-irradiated area. In the summer this difference was not significant. The possible implications of these findings are discussed.
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