Evaluation of Skin Reactivity: The concept of Histamine Equivalent Allergen Threshold Concentration (Cha)

Background: Skin prick testing is the most common diagnostic tool used by allergists. There are limited, international rules on interpreting and reporting skin test results in a meaningful way. Aim: This communication describes methods to express the results of skin prick tests in a meaningful way. It is recommended to use allergen extracts with defined composition, potency and stability, to keep the precision of SPT within acceptable limits by using duplicate tests, and to regularly calculate the c.v. If duplicate tests cannot be performed for practical and or psychological reasons, e.g. in small children, then it are proposed that regular proficiency tests are performed and reported. The method of estimating the allergen threshold concentration, histamine equivalent allergen concentration Cha, is described Conclusion: By adjusting the allergen wheal response to that of histamine, Cha, differences in techniques between personnel and centres can be minimized and changes in skin reactivity can be calculated as a threshold concentration. To document the skin reactivity and changes over time it is even proposed to report the mean histamine wheal response of groups and of testing personnel at all-time points of therapeutic trials and in practice over time.