Final Diagnosis -- Plasmacytoma with daratumumab interference presenting as a monoclonal IgG/kappa
Plasmacytoma with daratumumab interference presenting as a monoclonal IgG/kappa
This patient has a history of recurrent plasmacytoma. She has been closely followed up for disease recurrence and progression to MM. Upon identification of a new bone lesion at distal femur, which was not amenable to radiation therapy due to its close proximity to the knee joint, daratumumab was started as a single systemic therapeutic agent. A month later, a characteristic IgG/kappa band appeared on both SPE and IFE, most consistent with the presence of daratumumab instead of the development of a new clone. This should not be mistaken as the evidence of progression to IgG/kappa MM.
Daratumumab in patients' serum has been misidentified as the persistence of endogenous mAb or the development of a new clone. As the application of daratumumab and other therapeutic mAbs is growing, improving the awareness among clinicians and clinical laboratories that therapeutic mAbs can positively interfere with both SPE and IFE is imperative to prevent misdiagnosis. However, at present, there is no commercially available test to confirm the presence of daratumumab in patient samples. Therefore, the possibility that an endogenous serum IgG/kappa co-migrates with darartumumab cannot be theoretically excluded.
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Contributed by Li Liu, MD, PhD