The current study was conducted to determine the risk of adverse outcomes among patients with monoclonal gammopathy of undetermined significance (MGUS) of the IgM class. Two hundred thirteen patients with IgM MGUS were identified in southeastern Minnesota from 1960 to 1994. The primary end point was progression to lymphoma or a related disorder assessed by the Kaplan-Meier method. Patients were followed for a total of 1,567 person-years (median, 6.3 years per subject). Seventeen patients developed lymphoma (relative risk [RR], 14.8) and six progressed to Waldenstrom's macroglobulinemia (RR, 262), while three developed primary amyloidosis (RR, 16.3) and three others had chronic lymphocytic leukemia (RR, 5.7). The relative risk of progression was 16-fold higher in the IgM MGUS patients compared to the white population of the Iowa Surveillance, Epidemiology, and End Results (SEER) program. The risk of progression of MGUS of IgM type to lymphoma or related disorders averaged 1.5% per year throughout the period of observation.
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