A multicentre study for measuring skin hydration with 349 volunteers was carried out in six different laboratories. The purpose of the study was to investigate physical-, physiological- and product-dependent parameters of three test emulsions (base, base + moisturizer and base + moisturizer + lipids) in a double-blind study. A comparison between analogous and digital sensor technology of the Corneometer CM825 was examined. Here, a clear relationship between both sensor types could be highlighted. A vital point of the study was the division of the test subjects according to their skin type. To get more objective limits for three different skin types - very dry, dry and normal skin - visual expert evaluation, self-assessment and hydration measurements were analysed by means of statistical methods. The moisture-related skin types were determined as follows: very dry skin was characterized with corneometer units below 30, dry skin between 30 and 40 and normal skin higher than 40 a.u. (arbitrary units). The efficacy of the three test emulsions was examined in relation to the mentioned skin types. Analysing the measured data of all test centres, a clear dependency of skin physiology (skin type) and product efficacy became evident. The drier the skin, the higher the increase of hydration. The product performance of the three test emulsions compared to the untreated control resulted in a significant increase of skin hydration in all measuring centres. The evaluation of a product ranking showed a good differentiation between the basic emulsion and the two other products. An increase of efficacy by adding lipids could be observed in four of six centres. The important influence of the skin type of the volunteers on the degree of product performance, as demonstrated in this study, should be especially considered when drawing up guidelines for efficacy testing.

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