False-positive light chain clonal restriction by flow cytometry in patients treated with alemtuzumab

Objectives To increase awareness of potential diagnostic test interference associated with alemtuzumab, which is a therapeutic immunoglobulin G 1 monoclonal antibody used in hematologic malignancies, autoimmune diseases, and transplant-related disorders. Methods Bone marrow and blood from patients with T-cell prolymphocytic leukemia treated with alemtuzumab were evaluated by flow cytometry. Healthy donor blood was analyzed with or without in vitro treatment with alemtuzumab for comparison. Results Immunophenotypic analysis of bone marrow collected 4 weeks after alemtuzumab treatment demonstrated artifactual surface light chain restriction in CD19+ B cells and CD3+ T cells. Similar findings were observed in blood from another patient in a specimen collected 3 days after alemtuzumab treatment. These findings were recapitulated in healthy donor blood incubated with alemtuzumab. Conclusions Alemtuzumab can produce direct interference during flow cytometry analysis, resulting in false-positive evidence of light chain clonality. Clinicians and laboratorians should be cognizant of this risk to avoid misdiagnosis of B-cell neoplasms.