The basophil activation test confirms cow's milk allergy in non-sensitised children with allergy symptoms

International audienceBackground: The need for an oral food challenge (OFC) surrogate is growing in line with the continuous increase in the prevalence and severity of pediatric food allergy. The basophil activation test (BAT) is currently used for the diagnosis of venom and drug allergies. We made the hypothesis that BAT might improve the sensitivity of cow's milk (CM) allergy diagnosis in pediatric patients with negative skin prick tests (NSPT) and undetectable specific serum immunoglobulin E (USSIgE). Methods: BAT with CM extract (BAG-F2, Flow and Flow2CAST, Buhlmann Labs, Switzerland, final concentrations 25, 12.5, 5 and 1 ng/mL) was performed on freshly drawn blood from 50 consecutive CM allergic patients (0.5-14 years) with NSPT, USSIgE and positive CM oral challenge. For each donor, 3 controls were assessed: negative control (reaction buffer), positive controls (anti-IgE receptor and fMLP). BAT specificity was assessed with clinically irrelevant allergens in the study group and with CM extract BAG-F2 in non-allergic donors. BAT was monitored through flow cytometric membrane variations of CD63 expression and CCR3 downregulation. The percentage of CD63+ basophils and the mean fluorescence intensity of CD63 and CCR3 were used to define the basophil stimulation index. BAT was considered positive if at least 2 consecutive concentrations of CM extract induced a stimulation index of 2 or more compared with the negative control. Results: Cow's milk BAT was positive in 33 children and negative in 15. Two patients' results were not included in the final statistical analysis (one IgE-receptor non-responder and one rejection for technical reasons). BAT to irrelevant allergens in patients, as well as BAT to CM extract in nonallergic controls, were negative in 27 cases but positive for 3 BAT to CM in CM-tolerant children. BAT was subsequently used for the follow-up of 5 positive children, two of them over a period of 4 years, with results closely matching those of the oral food challenge. Conclusion: BAT brought the biological proof of food allergy in 33 out of 50 CM allergic children with NSPT and USSIgE. The sensitivity of the test was 69%, with 90% specificity, 90% positive predictive value and 64% negative predictive value. Clinical relevance: Cow's milk BAT proved to be a useful tool for the diagnosis and follow-up of CM allergy in children with NSPT and USSIgE. A positive CM BAT in such patients might therefore avoid OFC, but a negative test still needs OFC confirmation