Differential diagnosis by the endpoint method in patients skin-reactive to more than one inhalant allergen

In recent years there has been an increasing number of patients suffering from respiratory symptoms and skin test positive to more than one allergen, and in many cases neither the clinical history nor the specific IgE determination has proved useful in establishing a ranking among two or more positive allergens. This study was designed to test the usefulness of the skin-prick test endpoint technique for the identification of the most important allergen(s) among those skin test positive. To this end, 118 consecutive patients (age range 4-60 years) skin test positive to between 2 and 6 different allergens were selected and submitted to the skin-prick test endpoint technique with serial 1:4 dilutions of standardized commercial allergens. The skin-prick test endpoint technique was unable to establish a ranking in only 7 patients sensitized to two allergens, whereas a difference in the range 2-10 between the highest and the lowest endpoint dilution in the same patient could be established in all other cases. Some allergens known to be widely crossreactive (D. farinae and D. pteronyssinus; cultivated and common grasses; birch and hazel) showed a similar skin reactivity only in few cases when tested according to the skin-prick test endpoint technique. This finding suggests a specific sensitization only for one allergen (or mixture) of each crossreacting couple. Moreover, in 60/118 patients the allergen able to evoke the largest skin reaction with the standard diagnostic did not have the highest skin-prick endpoint, suggesting that the ranking of allergens on the basis of the dimension of the wheal obtained with the standard diagnostic is a concept that should be revised and updated.