Clinical Utility of Flow-Cytometry for Diagnosis and Genotype Phenotype Correlation in a Cohort of X-linked Agammaglobulinemia Patients

Excerpt

To the Editor: X-linked agammaglobulinemia (XLA) is one of the commonest inborn errors of immunity (IEI) with an estimated prevalence of 1:100,000 to 1:200,000 live births [1]. An early diagnosis facilitating timely initiation of intravenous immunoglobulin can prevent significant morbidity and mortality. Although identification of the underlying mutation in the BTK gene is considered gold standard for diagnosis, quantification of the intracellular BTK protein by flowcytometry provides a rapid tool facilitating the diagnosis [2]. We conducted a retrospective study among a cohort of 63 XLA patients diagnosed at our facility to evaluate flowcytometry’s precision in diagnosis of XLA as against the gold standard. The specificity of BTK expression by flowcytometry for absent or markedly reduced (less than or equal to 10%) expression was 100% for the diagnosis of XLA. It is important to note that though 95% patients typically present with either absent or diminished BTK expression, 5% may display normal expression but dysfunctional function [3]. In patients with partial/normal BTK expression, reduced mean fluorescence intensity (MFI) served as an important cue for diagnosis, thereby making the expression studies highly sensitive (96.5%) if both the protein expression and MFI are taken into consideration. We also observed that majority of our patients with absent or markedly reduced BTK expression had deletion or nonsense mutations, whereas patients having missense mutations had measurable or completely normal levels of the BTK protein. The levels of BTK expression also correlated with the severity of clinical manifestations. …