Inter- and intra-individual variability in human skin barrier function: a large scale retrospective study

In vitro transepidermal tritiated water flux measurements are frequently used to evaluate skin barrier integrity for quality control purposes. However, research in this area to date has been largely based upon small-scale studies, each involving relatively few skin permeation measurements. In order to enhance our understanding in this area, we have conducted a much larger scale retrospective statistical analysis of tritiated water kp values. These values reflected the permeability of 2400 skin samples that were derived from 112 female volunteers over a 4 year period. It was found that the population of tritiated water kp values constituted a positively skewed, non-Normal distribution. Mean kp was 2.04 × 10−3 cm/h while the 95th percentile was 4.50 × 10−3 cm/h. Both values are higher than those reported in previous smaller studies. Hence, our study indicates that previously suggested upper limits for tritiated water flux are too low and that they be revised upwards to a value of 4.5 × 10−3 cm/h. Analysis was also performed on smaller data subsets allowing inter-individual and intra-individual comparisons. For intra-individual kp variability, site-related differences yielded a non-Normal, positively skewed pattern in most individuals. Inter-individual variability was Normally-distributed and showed scatter that was much smaller in magnitude.