Artifactual Kappa Light Chain Restriction of Marrow Hematogones: A Potential Diagnostic Pitfall in Minimal Residual Disease Assessment of Plasma Cell Myeloma Patients on Daratumumab

Background Daratumumab (DARA) is a humanized Immunoglobulin G(IgG)1‐kappa monoclonal antibody against CD38 antigen that is shown to improve outcomes in relapsed/refractory plasma cell myeloma (PCM) patients. Since CD38 is expressed by different hematopoietic elements, DARA has the potential to interfere with flow cytometric assessment of bone marrow specimens. Methods Flow cytometric analysis of bone marrow samples from 10 PCM on DARA and 5 control samples was performed using two different antibody panels. Results Bone marrow samples from PCM patients on DARA exhibited a population of CD19+ CD10+ B‐lymphoid cells with kappa light chain restriction. Further morphological and immunophenotypic studies suggested that this population represents marrow hematogones. Marrow hematogones from control samples showed normal immunophenotypic profiles. Conclusion DARA on the surface of hematogones interferes with flow cytometric clonality study leading to artifactual kappa light chain restriction, which can result in false interpretation of a concurrent clonal B‐cell proliferation. In the era of rapidly growing list of therapeutic monoclonal antibodies, flow cytometry pathologists should be aware of potential interferences to avoid misdiagnosis. © 2019 International Clinical Cytometry Society