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Background: The basophil activation test is an emerging clinical tool in the diagnosis of cow's milk allergy (CMA). The aim was to assess the association between the basophil allergen threshold sensitivity to the major milk protein casein (casein-specific CD-sens), the levels of milk- and casein-specific Immunoglobulin E antibodies (IgE-ab), and the severity of allergic reactions at milk challenges.

Methods: We enrolled 34 patients aged 5–15 (median 9) years who underwent a double-blind placebo-controlled milk-challenge (DBPCMC) as screening before inclusion in an oral immunotherapy study for CMA. The severity of the allergic reaction at the DBPCMC was graded using Sampson's severity score. Venous blood was drawn before the DBPCMC. Milk- and casein-specific IgE-ab were analyzed. Following in vitro stimulation of basophils with casein, casein-specific CD-sens, was determined.

Results: Thirty-three patients completed the DBPCMC. There were strong correlations between casein-specific CD-sens and IgE-ab to milk (rs = 0.682, p <.001), and between casein-specific CD-sens and IgE-ab to casein (rs = 0.823, p <.001). There was a correlation between the severity of the allergic reaction and casein-specific CD-sens level (rs = 0.395, p =.041) and an inverse correlation between casein-specific CD-sens level and the cumulative dose of milk protein to which the patient reacted at the DBPCMC (rs = −0.418, p =.027). Among the 30 patients with an allergic reaction at the DBPCMC, 67% had positive casein-specific CD-sens, 23% had negative casein-specific CD-sens, and 10% were declared non-responders.

Conclusion: Two thirds of those reacting at the DBPMC had positive casein-specific CD-sens, but reactions also occurred despite negative casein-specific CD-sens. The association between casein-specific CD-sens and the severity of the allergic reaction and cumulative dose of milk protein, respectively, was moderate.