Laboratory Evaluation of Peripheral Blood Involvement in Mycosis Fungoides and Sézary Syndrome: Evolution of Flow Cytometry and Morphology Quantification and Interpretation

<b>Background/Objectives</b>: Mycosis fungoides (MF) and Sézary syndrome (SS) are cutaneous T-cell lymphomas (CTCLs) with variable clinical outcomes. Peripheral blood (PB) involvement in MF/SS is an independent predictor of prognosis. Accurate laboratory determination of PB involvement by MF/SS cells, however, is an ongoing challenge. Both flow cytometry (FC) and morphology-based quantification are limited by the overlap of CTCL cells and reactive T-cells. This study looks at the optimization over time of CTCL blood burden evaluation. <b>Methods:</b> This retrospective study reviews CTCL blood assessment at Northwestern Memorial Hospital from 2012 to 2021. Test ordering and reporting practices for morphology-based Sézary cell counts and FC were evaluated. For each assay, quantitative and qualitative results were analyzed and compared including percentages and absolute counts of abnormal T-cell populations and pathologist interpretations. <b>Results:</b> A total of 514 patients were evaluated, with increasing numbers of both tests ordered over time. FC quantitative metrics showed a moderate to high correlation with morphology metrics, especially for absolute CD4+/CD7- counts (correlation coefficient = 0.901, <i>p</i>-value < 0.001). Qualitative pathologist interpretations had moderate agreement between methods (kappa = 0.58). The recent addition of TRBC1 clonality assessment to our FC assay further optimizes the evaluation for CTCL blood burden. <b>Conclusions:</b> Flow cytometry offers a reliable approach for blood staging in MF/SS, and morphologic assessment may be redundant. This study provides a foundation for designing a new FC approach with TRBC1. This comprehensive review of the evolution of our laboratory practices may serve as a guide for other institutions with similar clinical needs.