The use of intradermal tests and relevance of negative controls in patients with negative or equivocal modified RAST test scores to inhalant allergens.

Patients with suspected inhalant allergy that demonstrate a Modified RAST Test (MRT) Class 0 and 1 (equivocal) may demonstrate positive infradermal (ID) skin whealing to suspect allergens. This whealing may be due, however, to an irritant reaction from glycerin used as a preservative in the extract. The purpose of this study is to establish the incidence of allergy as demonstrated by ID testing in patients with MRT Class 0 or 1 and to reinforce the importance of negative controls during ID testing. Fifty-five consecutive patients evaluated for allergy with MRT Classes 0 or 1 were studied. Intradermal testing with 1:500 wt/vol concentrations of allergenic extracts and a 2% glycerin control were performed. Thirty-eight patients (69%) were found to have ID whealing to allergenic extracts greater than the negative glycerin controls. We conclude that ID allergen testing of patients with MRT Class 0 or 1, when using a glycerin control, identifies additional allergens to be considered for immunotherapy.