Good agreement between minimal erythema dose test reactions and objective measurements: an in vivo study of human skin

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The erythema resulting from the minimal erythema dose (MED) test is subjectively assessed. The evaluator visually grades erythema on an ordinal scale. Both intra- and interobserver variation have been found for this erythema assessment. We wanted to examine if objective measurements could be used to confirm the subjective finding. One hundred two ultraviolet radiation (UVR)-exposed skin sites on the backs of 17 healthy volunteers were assessed. Erythema was visually graded according to a 5-point scale [0, (+), 1+, 2+, 3+] and objectively measured by a skin reflectance meter. Skin water content was objectively measured by tissue dielectric constant measurements. The relationship between subjective assessments and objective measurements of erythema was found to be linear (R(2) = 0.482, P < 0.0001). A positive correlation was found between subjective assessments of erythema and objective measurements of water content (Spearman's Rho = 0.414, P < 0.0001). Water content in categories 2+ and 3+ of the subjective erythema assessments differed significantly from the lesser categories (P < 0.0001). A linear relationship was found between the objective measurements of erythema and water content (R(2) = 0.241, P < 0.0001). Objective measurements of skin erythema and water content after UV exposure were in good agreement with the subjective assessments of erythema, but showed considerable interpersonal variation.