Xeroderma pigmentosum: the role of phototesting

We report three patients newly diagnosed as having xeroderma pigmentosum (X P). Previous photobiological studies have implicated ultraviolet (UV) B as the activating waveband in XP, causing a delayed and prolonged erythemal re sponse. This characteristic reaction pattern has been used as a quick scree ning test in patients suspected of having XP, while awaiting confirmatory D NA repair studies. Two of our patients showed no abnormal erythemal respons es, and one showed severe photosensitivity from 330 to 400 nm but normal UV B responses, with a peak erythema at 24 h. We conclude that the erythemal r esponses in XP are highly variable and cannot be considered as a reliable s creening test in the diagnosis of XP.