Atopic dermatitis is a clinical diagnosis based on ruling out parasites (Demodex species, fleas, ticks, lice, and mites), pruritic infections (bacterial and yeast overgrowth), flea-allergy dermatitis, and food allergy. Intradermal skin testing and in vitro blood allergy tests reflect exposure to allergens but do not answer the question, “Is my patient atopic?” In my practice, these tests are reserved for patients in which atopic dermatitis has been diagnosed and the owners want to pursue immunotherapy.
I generally start by performing a second-generation E-Screen (heska.com), a mini-screening serum allergy test. In addition to positive and negative controls, the test has 3 wells, 1 each for trees, weeds, and indoor allergens. If there is a positive reaction on this screening test, serum is submitted for a full screening. If the test result is negative, an intradermal skin test is performed.
A study in dogs showed that if the result of the E-Screen test was positive, the probability of obtaining a positive result from an in vitro allergy test or intradermal test was > 90%.1 In a recent study in cats, there was strong agreement between a positive E-screen result and a positive finding on in vitro allergy testing (88%).2