Factors influencing measurements

During the last two decades, an increasing number of noninvasive methods have been developed to determine skin properties in an objective way. The subjective, visual or tactile evaluation of skin conditions can now be quantified and numerical values can be obtained. These techniques are particularly useful in cosmetic testing. These types of methods can potentially detect and quantify some subclinical symptoms. However, standardization between instruments is at present imperfect and measuring the same skin property with different instruments can give different results. Instruments of different companies, even based on the same principle, use different scales and the transfer of data between laboratories is difficult. Therefore standardization, including calibration of the devices, becomes a key issue in applying these methods in efficacy testing, skin compatibility, mildness assessment, and in particular, in safety testing. Standardization is necessary on four different levels: environmental factors (room temperature, relative humidity, light sources, air circulation,.), instrumental variables (zero setting, calibration, probe properties, probe position,.), volunteer-linked factors (age, sex, race, anatomical site, diurnal rhythm, skin type, cleansing procedures, skin diseases, medication,.), and product-linked variables (galenic form, dilutions, amount per surface unit, frequency and mode of application, inclusion of blanks,.).