Evaluation of out-in skin transparency using a colorimeter and food dye in patients with atopic dermatitis

Summary Background  Atopic dermatitis is a disease of skin barrier dysfunction and outside stimuli can cross the skin barrier. Objectives  To examine a new method for evaluating the outside to inside skin transparency with a colorimeter and yellow dyes. Methods  In study 1, a total of 28 volunteer subjects (24 normal and four with atopic dermatitis) participated. After provocation with yellow dye, the skin colour of all the subjects was measured using a colorimeter. The skin transparency index was calculated by the changes of the skin colour to yellow. Other variables of skin function, including transepidermal water loss (TEWL) and stratum corneum hydration, were also measured. In study 2, the skin transparency index was evaluated for a cohort of 38 patients with atopic dermatitis, 27 subjects with dry skin and 29 healthy controls. Results  In study 1, the measurement of skin colour (b*) using tartrazine showed good results. There was a significant relationship between the skin transparency index with tartrazine and the atopic dermatitis score (P = 0·014). No other measurements of skin function, including the TEWL, were correlated. In study 2, the skin transparency index score obtained with tartrazine in the patients with atopic dermatitis was significantly higher than that of the controls and those with dry skin (P < 0·001 and P = 0·022, respectively). However, the TEWL in patients with atopic dermatitis was not significantly higher than that of patients with dry skin and the TEWL in subjects with dry skin was not higher than that of the controls. Conclusions  This method, which used a colorimeter and food dye, is noninvasive, safe and reliable for the evaluation of out–in skin transparency and can demonstrate the characteristic dysfunction in the skin barrier in patients with atopic dermatitis.