Smoldering multiple myeloma (SMM) is an asymptomatic clonal plasma cell disorder and is described as an intermediate stage between monoclonal gammopathy of undetermined significance (MGUS) and multiple myeloma (MM). In 2003, the International Myeloma Working Group (IMWG) classified patients with SMM as those who displayed an absence of end-organ damage or the signs and symptoms of amyloidosis typical of MM, as well as a bone marrow clonal plasma cell (BMPC) percentage of at least 10, a serum M protein level of at least 30 g/L, or a urine M protein level of at least 500 mg per 24 hours.
Patients with SMM face a higher risk of disease progression compared with those with MGUS but have a lower risk than patients with symptomatic MM. By definition, SMM is an asymptomatic condition, and the prognosis varies considerably. Overall, however, the risk of progression in SMM to symptomatic disease is approximately 10% during the 5-year period following diagnosis. After that, the risk is approximately 3% per year for years 6 through 10 and 1% per year after that. Ten years after diagnosis, the risk of disease progression becomes similar to that of MGUS.