CAR T Cell Expansion Masquerading as Plasmacytosis: Case Report of Significant Lymphocytosis After Cilta‐Cel in Multiple Myeloma

We report a case of delayed hypersensitivity dermatitis and an unusual lymphocyte expansion peaking at 45,000 cells/µL by Day +14 after ciltacabtagene autoleucel, a BCMA‐targeted CAR T therapy for multiple myeloma (MM). This marked post‐infusion lymphocytosis was initially mistaken for plasmacytosis but was confirmed by flow cytometry to be circulating CAR T cells. The lymphocytosis self‐resolved and was followed by complete remission. This case highlights that peripheral CAR T cell expansion may be mistaken for malignant plasmacytosis in the early post‐infusion period and underscores the critical role of flow cytometry in distinguishing CAR T cells from malignant plasma cells.