ABSTRACT
A malignant plasma cell clone usually produces a single abnormal monoclonal protein with a constant isotype. However, switching of paraprotein isotype has been reported to be a transient phenomenon associated with the recovery of B-cell function, and, in some cases, the switching might be misinterpreted as relapse. In August 2008, we encountered a case of a 59-year-old man with proteinuria and high IgG level (5.6 g/dL), kappa free light chain level of 2,660 mg/L, reversed A/G ratio (0.51), and multiple osteolytic lesions. Plasma cells, which accounted for 57% of all the nucleated cells, in bone marrow aspirates were positive for kappa immunostaining. Serum protein electrophoresis showed one M-spike, concentration of 4.87 g/dL in the beta region. Immunofixation electrophoresis revealed the peak as an IgG-kappa monoclonal protein; therefore, a diagnosis of plasma cell myeloma was made. Complete remission was achieved after chemotherapy, and autologous peripheral stem cell collection was performed. In March 2009, the patient underwent high-dose chemotherapy and autologous peripheral stem cell transplantation support. After 2 months, serum protein electrophoresis showed 2 M-spikes in the gamma region with positive IgM-lambda, IgG-lambda, and IgG-kappa, and these bands persisted. The electrophoretic mobility of the IgG-kappa protein was different from that of the original disease protein, and bone marrow results were the same as the previous ones. Although immunoglobulin isotype switch is known to have a benign course, it always requires careful monitoring because, in rare cases, true clonal switching may occur.