Cross-reactive carbohydrate determinants (CCD) cause multiple positive results in seasonal in vitro allergy tests. False positive/clinically irrelevant results have been identified due to the binding of immunoglobulin E against CCD (anti-CCD IgE) when testing for pollen allergens. The aim of this study was to investigate the prevalence of polysensitised serum samples and evaluate the impact of a CCD inhibitor/blocker in seasonal allergy test results.
 A total of 4614 canine serum samples, submitted from July 2017 to June 2018 for seasonal in vitro allergy test via ELISA Fc-Ε receptor technology, were studied. Samples were grouped into polysensitised (group A) and non-polysensitised (group B). Polysensitised samples were retested after adding a modified glycoprotein plant extract (blocker). To determine the impact of the blocker for each allergen, results prior and post blocking were investigated in 96 randomly selected samples.
 Polysensitisation to seasonal allergens was diagnosed in 818 (17.7 %) serum samples. The blocker eliminated the binding of anti-CCD IgE to allergen extracts (49 %) or suppressed the reaction classes (40 %) which are indicative of the presence of clinically relevant IgE. Negative reactions after blocking were less common when testing for antibodies against a mixture of 6-grass mix (29 %), rye (22 %), and sheep sorrel (20 %) in comparison to nettle (82 %), willow (70 %), birch-hazel (65 %), mugwort-ragweed (63 %) and English plantain (57 %).
 Blocking should be used in the case of polysensitized results to improve the quality of seasonal in vitro allergy tests and avoid the use of allergen-specific immunotherapy (ASIT) for clinically irrelevant allergens.

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