Monoclonal gammopathy of undetermined significance and smoldering multiple myeloma.

Patients with monoclonal gammopathy of undetermined significance (MGUS) have a monoclonal protein but no evidence of multiple myeloma, macroglobulinemia, amyloidosis, or related disorders. In a long-term follow-up (median, 19 years) of 241 patients with MGUS, 53 (22%) developed multiple myeloma (36 patients), macroglobulinemia (7 patients), primary systemic amyloidosis (7 patients), or malignant lymphoproliferative disease (3 patients). Fifty-seven patients (24%) remained stable and were alive, while 124 patients (51%) died of causes unrelated to the monoclonal gammopathy. Multiple myeloma was diagnosed in the 36 patients 23 to 251 months (median, 9.6 years) after recognition of the M-protein. Smoldering multiple myeloma (SMM) is characterized by a serum M-protein level greater than 30 g/l and greater than 10% plasma cells in the bone marrow but no anemia, renal insufficiency, or lytic lesions. Differentiation of MGUS and SMM from multiple myeloma and macroglobulinemia is difficult at the time of diagnosis of the M-protein, and the patients must be followed up indefinitely.