Abstract
A modified radioallergosorbent test (RAST) assay and scoring system (Fadal/Nalebuff RAST) have been purported to increase the diagnostic accuracy and sensitivity of the RAST. In order to assess such claims, we compared this modified RAST with the conventional Phadebas RAST in 40 patients with seasonal allergic rhinitis. Phadebas and modified RAST scores to nine allergens were highly correlated (r = +0.93) and results from either RAST correlated with skin-test titration end points. Clinical sensitivity to test allergens was assessed using nasal provocation titration; positive RAST determinations by both methods were associated with negative provocation tests in fewer than 10% of cases. However, positive provocation tests were noted in over half the patients who had negative Phadebas RAST results; the inclusion of an additional reference point at the lower end of the standard curve may increase the diagnostic accuracy of the test. Non-IgE-containing control preparations produced equivocal results in up to seven of 21 modified RAST assays, but produced negative results in all Phadebas RAST assays. Background binding produced by negative control preparations varied considerably among different allergen discs, indicating that the cutoff point between positivity and negativity may vary from allergen to allergen. The modified RAST was also used to calculate initial immunotherapy doses. Preliminary skin tests at these predicted allergen concentrations produced large wheals (>16 mm) in 15 of 50 trials (30%), and in this situation immunotherapy doses were withheld. The remaining 35 patients tolerated the initial immunotherapy doses without incident. As presently performed, the modified RAST assay does not afford significant diagnostic advantages over the Phadebas RAST; until such a time as standardized allergen extracts are available, potential therapeutic applications of the modified RAST remain speculative.
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