Utility of Flow Cytometry for Prognostic Prediction in Adult T‐Cell Leukemia/Lymphoma

Introduction Adult T‐cell leukemia/lymphoma (ATL) exhibits marked clinical heterogeneity; however, current classification primarily rely on morphology and laboratory findings and require further advancement. This study aimed to evaluate the roles of human T‐lymphotropic virus type 1 (HTLV‐1)–infected cell (CD4‐positive T cell) analysis system using flow cytometry (HAS‐Flow) and absolute CD4 count in prognostic assessment of nonaggressive ATL. Methods We retrospectively analyzed 227 individuals with HTLV‐1 infection in Japan (derivation cohort) and 81 in the United Kingdom (validation cohort). HAS‐Flow categorized patients into G1–G4 groups according to the proportion of cell adhesion molecule 1 (CADM1) + /CD7 − cells. Clinical outcomes were compared by HAS‐Flow and absolute CD4 count (cutoff: 1500 cells/µL) using Kaplan–Meier and Cox analyses. Results In the derivation cohort, 4‐year progression‐free survival (PFS) rates were 100% for G1–G2, 94.2% for G3, and 55.7% for G4 ( p < 0.01). Absolute CD4 counts >1500 cells/µL identified patients with significantly shorter PFS and overall survival. G4 patients showed poor prognosis irrespective of CD4 counts. The UK validation cohort reproduced the prognostic CD4 counts >1500 cells/µL. Conclusion HAS‐Flow and absolute CD4 count provide additive prognostic information beyond the Shimoyama classification. Trial Registration The authors have confirmed clinical trial registration is not needed for this submission.