Total IgE titre does show some correlation with disease activity and it can be a useful screening tool in patients in whom a diagnosis of atopic dermatitis is being considered. Positive radioallergenosorbent (RAST) tests to foods such as nut, wheat and milk are rare in patients with atopic dermatitis, but in Northern European affected adults there are positive RAST tests to:
▪ house dust mite in 50-60%
▪ cat dander in 50%
▪ grass in 75%
▪ birch pollen in 65%.
There is generally a good correlation between skin prick test reactivity and RAST test to these allergens.
Prick tests involve the intradermal injection of allergen with assessment of the immediate urticarial reaction and are believed to reflect type I allergic reactions. In contrast patch tests involve applying antigen to the skin for 48 hours and assessing the subsequent delayed eczematous reaction. This is believed to more closely reflect type IV delayed hypersensitivity reactions and is used clinically to investigate patients with suspected contact allergic dermatitis to environmental antigens such as nickel.
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