Variation in stratum corneum protein content as a function of anatomical site and ethnic group

Objectives: Quantification of stratum corneum (SC) protein levels from tape strippings is frequently used to investigate skin conditions, to correct for amounts of SC protein removed in SC biomarker studies and to determine distribution of topically applied ingredients. In recent years, a rapid and convenient method for SC protein quantification from tape strippings has become available using infrared densitometry (IRD). However, standard curves have only been generated for Caucasian forearm and shoulder SC and have been assumed to be correct not only for facial SC but also for SC samples of other ethnic groups. The aim of this study was to investigate whether the use of IRD for SC protein measurement is valid for other body sites such as the cheek and for measuring SC protein content of darkly pigmented skin types. 

Methods: Ten Caucasian and ten Black African female subjects with self-assessed normal skin participated in the study. Tape strippings were collected from two different body sites (forearm and cheek). First from the tape strippings, the SC optical absorption was determined densitometrically. This obtained absorption (%) was compared with absolute SC protein extracted from the same tapes using a colorimetric microbicinchoninic acid (μBCA) assay. 

Results: Higher amounts of SC protein were removed from the forearm compared with the cheek (P < 0.01). The absolute SC protein concentration quantified by μBCA assay and the absorption of SC proteins by IRD followed a similar profile. There was no significant difference found between the two ethnicities in SC protein (P > 0.05). The overall coefficient of determination (R²) shows a good fit to the regression line between the two methods in both sites (forearm = 0.82, cheek = 0.77). Also, both ethnicities showed good correlation (R² ≥ 0.69, P = 0.01). 

Conclusions: Facial SC is morphologically distinct from the forearm, as demonstrated by the differences in amounts of SC removed. Although the data distribution in different subject groups varied, the regression was always quite similar between the two body sites and both ethnic groups. Also, the correlations were similar to previously published data on other body sites. The resultant calibration curves can be used as a rapid indirect protein assessment of tape strippings from the cheek.