A diagnostic pitfall in cold agglutinin disease: KMT2D-mutated CAD-associated lymphoproliferative disorder with a CLL-like immunophenotype.

Cold agglutinin disease (CAD) is an uncommon hemolytic anemia by complement-fixing IgM autoantibodies and is frequently associated with a distinct clonal B-cell lymphoproliferative disorder (CAD-LPD). We report a woman in her 50's with progressive fatigue who presented with hemolytic anemia and an IgM-κ paraprotein. Bone marrow examination revealed a small clonal B-cell population (5.8%) with a CLL-like immunophenotype. Despite the absence of peripheral lymphocytosis or established treatment indications for chronic lymphocytic leukemia (CLL), B-cell-directed therapy, including ibrutinib and venetoclax plus rituximab, was administered over the course of approximately one year based on the initial diagnostic impression, but failed to control hemolysis. Targeted next-generation sequencing identified a KMT2D frameshift mutation, supporting a diagnosis of CAD-LPD rather than monoclonal B-cell lymphocytosis. Subsequent treatment with the C1s inhibitor sutimlimab resulted in sustained resolution of hemolysis. This case highlights diagnostic pitfalls in CAD and underscores the value of molecular profiling in guiding appropriate therapy.