Usefulness of skin phototype as a parameter of phototoxicity after oral or topical administration of psoralen in patients before PUVA therapy

Introduction. The assessment of cutaneous sensitivity to UV radiation is necessary prior to phototherapy. The classification of phototype proposed by Fitzpatrick in 1975 is a method still used in many dermatology centres. The patient’s skin phototype depends on the individual past history of solar induced erythema and tanning after 30 minutes of first summer sun exposure and has proved useful for estimating the initial dose of ultraviolet radiation in phototherapy. Since that time many papers on the relation between skin phototype and skin sensitivity to ultraviolet radiation have been reported. objective. The purpose of this study was to assess the usefulness of skin phototype evaluation as a parameter of UVA sensitivity after oral or topical administration of psoralen in patients before PUVA therapy.
Material and methods. Three hundred and seventeen persons, aged 10-78 years, were enrolled into the study. The skin phototype was recorded according to Fitzpatrick’s classification. The MPD was determined in 281 patients by phototesting 2 h after ingestion of 5-MOP, given in standard doses of 1.2-1.4 mg/kg or directly after 20-min 8-MOP bath (1 mg/l, 37°C) in 36 subjects. Parametric and non-parametric statistical tests were used.
Results. There was a significant correlation between skin phototype and MPD value. Mean MPD value was 2630 mJ/cm² (range from 1000 to 6000 mJ/cm²). Our results demonstrated a very wide range of MPD within subjects with identical skin type and a considerable overlap in the MPD values among neighbouring skin phototypes. The MPD determined after ingestion of 5-MOP was significantly higher than after 8-MOP bath. Age and gender did not substantially influence the results of phototesting. There was no significant difference in results of phototests in patients before phototherapy and those in healthy volunteers.
Conclusion. Our results indicate significant individual differences in the MPD values in patients with identical skin types, which indicates the necessity of phototesting before phototherapy. Fitzpatrick’s classification of skin phototype alone is not enough to establish the initial dose of UVA before photochemotherapy.