IN PREVIOUS studies* a method has been developed for the measurement of pruritus. The most satisfactory itch stimulus was found to be a freshly prepared histamine phosphate solution in a special diluent.† A quantitative technique was devised so that the end-point of subjective reactivity could be measured. This end-point of reactivity was called the itch threshold and was defined as the greatest dilution of histamine which produced a recognizable pruritus. The itch threshold was then utilized as a tool for the study of physical, environmental, and psychological factors which have been thought to influence the development and severity of itching.
An assessment of the limitations of the test was deemed advisable. Accordingly, extensive testing in control subjects was performed. It was found that the itch threshold varied greatly from person to person, with extremes of reactivity between dilutions of 1:1,000 and 1:800,000,000 and an average reactivity in the 1