The use of Flow Cytometry in Diagnosing X-linked Agammaglobulinemia and Genotype-Phenotype Correlation

X-linked agammaglobulinemia (XLA) is a prevalent inborn error of immunity (IEI), where early diagnosis is crucial for timely intervention with intravenous immunoglobulin to prevent severe health outcomes. While identifying mutations in the BTK gene remains the gold standard for diagnosing XLA, flow cytometry to quantify intracellular BTK protein offers a rapid diagnostic alternative.

In a study involving 63 XLA patients, flow cytometry demonstrated high precision in diagnosing XLA. Specifically, the test showed 100% specificity for diagnosing XLA when BTK expression was absent or markedly reduced (≤10%). Although 95% of patients typically exhibit absent or diminished BTK expression, about 5% may show normal levels with dysfunctional function. For these cases, reduced mean fluorescence intensity (MFI) indicated a diagnosis, making the method highly sensitive (96.5%) when both protein expression and MFI are considered.

Patients with deletion or nonsense mutations generally had absent or significantly reduced BTK expression, while those with missense mutations displayed measurable or normal BTK levels. BTK expression levels correlated with the severity of clinical symptoms.

In conclusion, evaluating BTK expression by flow cytometry is recommended as the initial diagnostic tool for XLA in patients with less than 2% B cells and hypogammaglobulinemia. This method is also effective for detecting female carriers due to the distinctive bimodal BTK protein expression pattern.

Yadav RM, Desai SS, Gupta M. et al. Clinical Utility of Flow-Cytometry for Diagnosis and 1Genotype-Phenotype Correlation in a Cohort of X-linked Agammaglobulinemia Patients. Indian J Pediatr. 2024; 91, 638. https://doi.org/10.1007/s12098-024-05127-9