Immediate allergic responses, orchestrated by basophils and mast cells, are pivotal in severe allergic reactions. The flow cytometry-based Basophil Activation Test (BAT) is a clinically important assay for testing allergic reactions using CD63 and CD203c as endpoints. The test measures the concentration dependent response to the allergens providing a functional readout of the patients' allergies. BAT is presently in clinical use within the Unites States as well as several other countries for the diagnosis and monitoring of allergies, most commonly against food allergens. This article details assay validation, both analytical and clinical with reference to existing regulations/recommendations through CAP, CLIA, NYS and CLSI on issues including accuracy, precision, linearity, reportable range, reference range, analytical sensitivity & specificity, pre-analytical considerations, the utility of the assay in diagnosis or monitoring and interpretation of the results
and the assay's limitations. The BAT plays a crucial role in assessing patient suitability for food challenges and therapies such as oral immunotherapy, sublingual immunotherapy or omalizumab and can aid in predicting treatment outcomes. We further review the current research on advancing the test, focused on improvements in its clinical utility. Continuous efforts are warranted for enhanced regulatory oversight and comprehensive clinical validation, ensuring BAT's seamless integration into diverse clinical settings.