Instrumental Measurement of the Mantoux Test: Differential Effects of Tuberculin and Sodium Lauryl Sulphate on Impedance Response Patterns in Human Skin

Background and Objective: The tuberculin (PPD) test is a model reaction for the cell-mediated immune system of the skin. The aim of this study is to compare the non-invasive technique based on electrical impedance for investigation of the tuberculin test with an irritant contact reaction induced by sodium lauryl sulphate, which has already been carefully evaluated using this method. Methods: 0.1 ml of PPD (2 TU) was injected intracutaneously into the volar forearms of 20 adult healthy subjects, all known to be tuberculin sensitized. Assessments were performed 3 and 7 days after the injections, using visual scoring, a new electrical impedance technique and transepidermal water loss (TEWL). Results: In the 16 cases that completed the study, compared to relevant controls and reactions induced by sodium lauryl sulphate statistically significant changes in 3 of the 4 impedance indices were found: both the means of index MIX and index IMIX increased (p ≤ 0.01) and the mean of index PIX decreased (p ≤ 0.05). Furthermore, similar impedance response patterns occurred in another cell- mediated immune reaction that we have studied previously – namely, the allergic contact reaction induced by nickel sulphate. Unlike the irritant reactions, no significant increase in TEWL values in tuberculin reactions were found. This can be explained by the fact that the epidermis is little affected, since the inflammatory process in the tuberculin reaction is located more deeply in the dermis than with the irritant reaction. Conclusion: The current study strongly indicates that electrical impedance in the described version, besides contact dermatitis, also seems suitable for studies of the tuberculin reaction. This will add a new indication for the technique, and studies of further clinical applications are in progress.