Monoclonal gammopathy of undetermined significance.

Monoclonal gammopathy of undetermined significance (MGUS) accounted for 56% of the 1026 patients with a monoclonal plasma cell disorder seen at the Mayo Clinic during 1992. Approximately 3% of persons older than 70 years and 1% of those older than 50 years had MGUS. In a series of 241 Mayo Clinic patients with a serum M-protein but no evidence of multiple myeloma, macroglobulinemia, primary amyloidosis, lymphoma, or related disorders, followed up for 20 to 35 years (median, 22 years), the condition of 19% remained stable. Ten percent had an increase in the serum M-protein to 3.0 g/dL or more but did not require chemotherapy, whereas 47% died of unrelated causes. 59 of the 241 patients (24.5%) developed a serious disease during the median follow-up period of 22 years: multiple myeloma in 39 patients, primary amyloidosis in 8, Waldenström's macroglobulinemia in 7, and other malignant lymphoproliferative disorders in 5. The actuarial rate of malignant transformation was 17% at 10 years and 33% at 20 years. The median interval from diagnosis of MGUS to the diagnosis of serious disease was 8.5 to 10.5 years. No single factor can differentiate a patient with benign monoclonal gammopathy from one in whom a malignant plasma cell disorder will subsequently develop. Therefore, the serum M-protein must be measured and a clinical evaluation conducted periodically.