Intensity and area increase of UVB-induced erythema: two variables used for studies of the influence of topically applied drugs.

The area of UV erythema produced by a small beam head was found to increase with increasing doses. The aim was to investigate whether measurement of the area could be a more useful indicator of UV-induced damage than classic visual grading. Topical pretreatment with all-trans retinoic acid (tretinoin) and betamethasone valerate was used to test the applicability of the method in pharmacological studies. We used a round outlet head (5 mm2) connected by optical fibre to a monochromatic irradiator, and doses ranging from 0.05 to 0.2 Joule of 300 mm UV light were applied to the skin of 6 healthy subjects. Erythemal area was calculated by the measurement of two diameters, and intensity was graded visually (0-6 scale). The area of the erythema correlated with the increase in intensity up to score 6. Area measurement was less subject to intra-investigators' variability than the intensity score. By multiplying intensity by area, a good indicator of UV-induced reactivity was obtained. Pretreatment with betamethasone valerate decreased the area of erythema, as did tretinoin 12 h after irradiation. Thus, area measurement of erythema is a useful adjunct to visual grading of UV-induced skin reactions.