Introduction: The Unmet Need in Cow’s Milk Allergy Management
Cow’s milk allergy (CMA) remains the most prevalent food allergy among pediatric populations globally and stands as a leading cause of fatal food-induced anaphylaxis. For clinicians, the primary challenge lies in the inherent unpredictability of allergic reactions. While skin prick tests (SPT) and serum-specific IgE (sIgE) measurements are standard diagnostic tools, they are often excellent at confirming sensitization but poor at predicting the clinical phenotype—specifically, the severity of a reaction and the threshold of allergen exposure required to trigger it.
Currently, the gold standard for determining these parameters is the Oral Food Challenge (OFC). However, OFCs are resource-intensive, carry a significant risk of severe anaphylaxis, and cause substantial anxiety for patients and caregivers. The identification of a reliable, non-invasive biomarker that can risk-stratify patients before they undergo an OFC is a critical priority in precision allergy medicine. The BAT2 study, recently published in Allergy, provides compelling evidence that the Basophil Activation Test (BAT) may be the solution to this long-standing clinical dilemma.
Study Design and Methodology
The BAT2 study (NCT03309488) was a prospective, longitudinal investigation designed to evaluate the utility of various immunological biomarkers in predicting the outcomes of OFCs to both baked milk (BM) and fresh milk (FM). The study population included 71 children who exhibited clinical reactions to cow’s milk protein.
Participants underwent standardized OFCs according to the Practall guidelines, which are recognized for their rigorous methodology in assessing food allergy. The researchers evaluated demographic data, clinical history, and a suite of immunological parameters, including SPT wheal size, sIgE levels, and the Basophil Activation Test (BAT). The BAT measures the expression of activation markers, such as CD63, on the surface of basophils following ex vivo stimulation with milk proteins. Receiver Operating Characteristic (ROC) curve analyses were utilized to determine the diagnostic accuracy and optimal cut-off points for each biomarker in identifying severe reactors and low-threshold reactors.
Key Findings: BAT as a Unique Predictor of Clinical Phenotype
The results of the BAT2 study highlight a significant divergence between the predictive power of BAT and traditional testing modalities. Of the 71 children who reacted during the challenges, 22 (15%) reacted to baked milk and 49 (43%) reacted to fresh milk. Among these reactors, 32% of those reacting to BM and 24% of those reacting to FM experienced severe symptoms.
The Only Biomarker for Severity and Threshold
The most striking finding was that the Basophil Activation Test was the only biomarker capable of distinguishing between severity and threshold groups. While sIgE and SPT could indicate the presence of an allergy, they failed to correlate significantly with the intensity of the reaction or the dose at which the reaction occurred.
Predicting Baked Milk Severity
For children undergoing baked milk challenges, identifying those at risk for severe reactions is vital, as baked milk is often used as a first step in oral immunotherapy (OIT) or dietary reintroduction. The study found that BAT optimal cut-offs achieved a sensitivity of 71% and an impressive specificity of 100% in identifying severe reactors to BM. This suggests that a positive BAT result at a specific threshold can almost certainly identify a child who will have a severe reaction, allowing clinicians to bypass high-risk challenges or proceed with extreme caution.
Determining Fresh Milk Thresholds
For fresh milk, the focus shifted to the reactivity threshold. The median cumulative dose of milk protein tolerated was 0.143 g for FM. BAT cut-offs demonstrated a sensitivity of 96% and a specificity of 41% for identifying children who would react to this low dose or less. This high sensitivity makes BAT an excellent screening tool to identify highly sensitive patients who are at risk of reacting to trace amounts of milk in their environment.
Mechanistic Insights: Why BAT Outperforms Other Tests
The superiority of BAT in this context likely stems from its nature as a functional assay. Unlike sIgE, which merely measures the concentration of circulating antibodies, BAT reflects the actual biological response of effector cells. Basophils are sensitized with IgE on their surface; the BAT assay measures the degree of degranulation and activation when these cells encounter the allergen in a controlled environment. This process accounts for factors such as IgE affinity, the presence of blocking antibodies (like IgG4), and the intrinsic releasability of the patient’s effector cells—variables that serum tests cannot capture.
Expert Commentary and Clinical Implications
The ability to accurately risk-stratify CMA patients has profound implications for clinical practice. If integrated into standard care, BAT could significantly reduce the number of unnecessary or high-risk OFCs. For patients identified as “severe reactors” via BAT, clinicians might opt for earlier intervention or more stringent avoidance strategies. Conversely, identifying “low-risk” or “high-threshold” patients can provide reassurance to families and facilitate a faster progression through the milk ladder.
However, limitations must be acknowledged. BAT requires fresh blood samples and specialized laboratory equipment (flow cytometry), which may limit its availability in non-academic or resource-limited settings. Furthermore, while the specificity for BM severity was perfect in this cohort, the relatively lower specificity for FM thresholds (41%) suggests that while BAT is excellent at catching sensitive reactors, it may produce some false positives regarding low-threshold sensitivity.
Conclusion: A Shift Toward Precision Diagnostics
The BAT2 study establishes the Basophil Activation Test as a cornerstone of precision diagnostics in cow’s milk allergy. By providing data on both the severity of potential reactions and the threshold of reactivity, BAT offers a level of granular detail that traditional testing lacks. As this technology becomes more widely accessible, it is poised to improve patient safety, reduce the burden of oral food challenges, and personalize the management of children living with this challenging condition.
Funding and Clinical Trial Information
This research was conducted as part of the BAT2 study, registered at ClinicalTrials.gov under the identifier NCT03309488. The study was supported by various clinical research grants and institutional funding dedicated to improving food allergy diagnostics. No conflicts of interest were reported that compromised the integrity of the data.
References
1. Boyd H, Bartha I, Foong RX, et al. Basophil Activation Test as Biomarker of Severity and Threshold of Allergic Reactions to Cow’s Milk During Oral Food Challenges. Allergy. 2025 Dec 18. doi: 10.1111/all.70175.
2. Santos AF, Du Toit G, Douiri A, et al. Distinctive role of basophil activation test in predicting outcomes of oral food challenges. J Allergy Clin Immunol. 2014;134(3):646-652.
3. Sampson HA, Gerth van Wijk R, Bindslev-Jensen C, et al. Standardizing double-blind, placebo-controlled oral food challenges: American Academy of Allergy, Asthma & Immunology-European Academy of Allergy and Clinical Immunology PRACTALL consensus report. J Allergy Clin Immunol. 2012;130(6):1260-1274.