Critical evaluation of the use of skin tests and cellular tests in standardization of allergens.

In two groups of 8 and 40 allergic patients repeatedly skin-tested with various dilutions of timothy allergen and with histamine (10 mg/ml) intradermally and by prick test, the skin reactions were evaluated by wheal and erythema size at 20 minutes and kinetically followed over 60 minutes by infrared telethermography. The latter technique permits to define further reaction parameters, such as thermographic area and Thermographic Units based on the average elevation of temperature in the reaction site. These studies have led to several conclusions. First, the skin area really involved in skin reactions to allergens or to histamine are much larger than assessed visually. There is more to it than meets the eye. Second, among all parameters studied in terms of reproducibility upon repeated testing, erythema and thermographic areas are the most stable, while wheal area shows considerably larger variations. Third, there is no good correlation between reactions to allergen dilutions and reactions to histamine, either individually or as groups of patients. In addition, the ratio of thermographic area (as true indication of allergic inflammation) to erythema area and the kinetics of the reaction are very different between allergen-induced and histamine-induced reactions. There is therefore no real advantage in relating reactions to allergen to reactions to histamine, in terms of biological standardization. Fourth, the intradermal technique, in terms of reactions to histamine, definitely shows better reproducibility than the prick test technique in skilled hands. A method such as the HEP standardization technique, combining prick testing, evaluation of skin reactions by wheal area and reference to a histamine standard is therefore the worst of possible alternatives. The US method, based on intradermal injection of various dilutions of allergen and evaluation of erythema, seems to us more adequate and reproducible for quantitative biological standardization of allergen extracts by skin testing. This statement does not mean that prick testing could not nevertheless be considered as a method of choice for diagnostic skin testing.