An assessment of the role of intradermal skin testing in the diagnosis of clinically relevant allergy to timothy grass

Background: Immediate skin resting is generally the preferred method for establishing the presence of allergy in clinical practice. There is no agreement however as to whether intradermal testing should be routinely performed if skin prick test results are negative. Purpose: The study was done to address the value of intradermal skin testing in the diagnosis of clinically significant sensitivity to grass pollen in patients exhibiting negative skin prick test responses to timothy extract. Methods: Four groups were studied. Group I had a history of seasonal allergic rhinitis, negative skin prick test responses to timothy and Bermuda grass, but positive intradermal skin test responses to timothy grass. Group II had a history of seasonal allergic rhinitis and positive skin prick test responses to timothy grass. Group III had a history of seasonal allergic rhinitis but had negative responses to both prick and intradermal testing with timothy and Bermuda grass. Group IV had no history of rhinitis, had negative responses to skin testing with a panel of locally important allergens, as well as Bermuda and timothy grass, and had a serum IgE value of less than 20 IU/ml. Clinical sensitivity to grass was assessed by two methods: (1) nasal challenge with threefold increasing amounts of timothy pollen performed out of the pollen season and (2) correlation of subjects' daily symptom and medication scores with daily grass pollen counts during the grass pollen season. Results: On the basis of nasal challenge with timothy grass, pollen allergic reactions were present in 11% of group I, 68% of group II, 11% of group III, and 0% of group IV. As determined by correlation of symptoms during the grass pollen season with grass pollen counts, 22% of group I, 64% of group II, 21% of group III, and 0% of group IV were considered allergic. If both criteria were required for a diagnosis of clinical allergy to grass, the percent positive was 0 for group I, 46 for group II, 0 for group III, and 0 for group IV. Conclusion: Under the conditions of this study the presence of a positive intradermal skin test response to timothy grass (1000 AU/ml) in the presence of a negative skin prick test response to timothy grass (100,000 AU/ml) did not indicate the presence of clinically significant sensitivity to timothy grass, and by inference, to other cross-reacting grasses.