Differentiation of allergic and irritant reactions by transepidermal water loss

12 patients with known allergy to nickel were patch tested with nickel sulphate, nickel chloride, and sodium lauryl sulphate 1, 2, 5 and 10%. 2 parallel series were performed, i.e., one with 24-h and one with 48-h application time. Reactions were followed by measurement of transepidermal water loss (TEWL). Allergic reactions showed a normal TEWL after 24-h application, and after 48 h, only strong reactions resulted in increased TEWL. In contrast, irritant reactions showed increased TEWL in both series, irrespective of the strength of the reaction. The deficient water barrier of allergic reactions is secondary, probably resulting from the underlying inflammatory process. Consequently, if TEWL measurements are used for the purpose of differentiating allergic and irritant skin reactions, a short application period, i.e., in this study 24 h, is preferable, with recordings performed shortly after removal of test chambers.