Smoldering (asymptomatic) multiple myeloma is an asymptomatic plasma-cell proliferative disorder associated with a high risk of progression to symptomatic multiple myeloma or amyloidosis. Prognostic factors for the progression and outcome of this disease are unclear.

We searched a computerized database and reviewed the medical records of all patients at Mayo Clinic who fulfilled the criteria of the International Myeloma Working Group for the diagnosis of smoldering multiple myeloma between 1970 and 1995. Bone marrow aspirate and biopsy specimens were studied, and patients were followed throughout the course of disease.

During the 26-year period, 276 patients fulfilled the criteria for smoldering multiple myeloma. During 2131 cumulative person-years of follow-up, symptomatic multiple myeloma or amyloidosis developed in 163 persons (59%). The overall risk of progression was 10% per year for the first 5 years, approximately 3% per year for the next 5 years, and 1% per year for the last 10 years; the cumulative probability of progression was 73% at 15 years. At diagnosis, significant risk factors for progression included the serum level and type of monoclonal protein, the presence of urinary light chain, the extent and pattern of bone marrow involvement, and the reduction in uninvolved immunoglobulins. The proportion of plasma cells in the bone marrow and the serum monoclonal protein level were combined to create a risk-stratification model with three distinct prognostic groups.

The risk of progression from smoldering multiple myeloma to symptomatic disease is related to the proportion of bone marrow plasma cells and the serum monoclonal protein level at diagnosis.