Evaluation of T‐cell clonality by anti‐TRBC1 antibody‐based flow cytometry and correlation with T‐cell receptor sequencing.

Summary: Flow cytometry (FC) incorporating the T‐cell receptor β constant chain‐1 (TRBC1) has been recently proposed as a new standard in T‐cell clonality assessment. While early studies demonstrated high sensitivity in samples with conspicuous tumour burden, performance in real‐world samples, including those with low tumour burden and correlation with molecular methods has been limited. We evaluated TRBC1‐FC performance and correlated the results with high‐throughput TRB sequencing and a targeted next‐generation sequencing gene panel. Our cohort consisted of 90 evaluable samples from 57 patients. TRBC1‐FC confirmed T‐cell clonality in 37 out of 38 samples (97%) that were involved in a mature T‐cell neoplasm (MTCN). T‐cell clonality was also identified in nine samples from patients lacking a current or prior diagnosis of MTCN, consistent with the emerging entity T‐cell clonality of uncertain significance. TRBC‐FC was polyclonal in all samples and negative for disease involvement by standard pathology assessment. However, correlation with TRB sequencing in 17 of these samples identified two cases that harboured the known clonal sequence from index testing, indicating the presence of measurable residual disease not otherwise detected. Our study provides real‐world correlative validation of TRBC1‐FC, highlighting the strengths and limitations pertinent to its increasing implementation by general diagnostic laboratories. [ABSTRACT FROM AUTHOR]