Visual assessment of human skin irritation: a sensitive and reproducible tool

Human volunteer studies of skin irritation have been carried out for decades, both for research and for safety evaluation purposes. For the majority of this time, and consequently for the majority of the studies, assessment of the skin reactions has been made visually. Typical endpoints include erythema, oedema, dryness and scaling, some or all of which would be rated on a simple scale, eg 0, +/-, +, ++, + + +. Such approaches can be criticized as subjective, of poor reproducibility, lacking in sensitivity and highly variable between observers and/or institutions. In consequence, instrumental methods of assessment have been strongly promoted and do indeed offer several advantages, not least their objectivity. However, it is possible to use the human eye, which is a very sensitive tool, to make detailed, accurate and reproducible descriptions of skin irritation reactions. To achieve this, it is necessary to give prolonged and thorough training to each observer. In this paper, 3 examples of human volunteer studies, in which different pairs of trained observers independently carried out double blinded scoring of the irritation reactions, are reported. The grading patterns produced were almost identical; statistical analysis showed that properly trained observers are in fact able to reliably measure a grade of erythema to within +/- 1 on a 10 point scale; 97.6% of scores were within 2 grade points on this scale. These results provide evidence that visual scoring can be sensitive, reliable and reproducible within a testing institution.