Comparison of human back versus arm skin region for its suitability to test weak irritants

Large-scale comparative screening of der-matological and cosmetic products require non-invasive bioengineering methods which give reproducible results that can be statistically analyzed. To determine the most suitable test region for testing irritating substances, the response of the skin to the weak irritant sodium lauryl sulphate (SLS) was measured on standardized locations on the forearm and back. Furthermore, using 3 bioengineering methods, we aimed to establish the most sensitive method to measure skin response to SLS.

The baseline measurements of transepidermal water loss (TEWL), cutaneous blood flow (CBF) and skin colour a*-value were performed at 6 standardized locations on the volar forearms and at 12 locations on the back on 2 consecutive days, day 1 and 2, in 9 healthy volunteers. On day 2, 9 out of the 18 sites were exposed to 0.5% SLS by occlusive 24-h patches and the skin response was evaluated on days 3 and 4.

For TEWL, CBF and a*-value, no significant regional variations were found within individuals at any time on the arm locations and on the back locations. However, significant inter-individual variation was found. At the back on day 4, a slightly higher skin response to SLS was found compared to that found on the forearms as measured by TEWL and the a*-value. Of the 3 bioengineering methods used, TEWL was the most sensitive method to detect cutaneous response to the weak irritant used. TEWL measurements on days 1 and 2 performed at the 6 standardized locations on the volar forearms inside and outside a specially constructed air convection protection box, showed no relevant differences in measured TEWL values.

Both the forearms and the back were found to be suitable sites for irritancy testing. Since considerable differences in mean reactivity between subjects have been observed the use of large numbers of subjects in predictive irritant patch testing is required.