Monoclonal gammopathy of undetermined significance (MGUS , unknown or uncertain may be substituted for undetermined), formerly benign monoclonal gammopathy, is a condition in which a paraprotein is found in the blood during standard laboratory tests. It resembles multiple myeloma and similar diseases, but the levels of antibody are lower, the number of plasma cells (white blood cells that secrete antibodies) in the bone marrow is lower, it has no symptoms or problems, and no treatment is indicated. However, multiple myeloma develops at the rate of about 1-2% a year, so doctors recommend monitoring it yearly. In rare cases, it may also be related with a slowly progressive symmetric distal sensorimotor neuropathy.
MGUS is a common, age-related medical condition characterized by an accumulation of bone marrow plasma cells derived from a single abnormal clone. Patients may be diagnosed with MGUS if they fulfill the following four criteria:
A monoclonal paraprotein band lesser than 30 g/L (< 3g/dL);
Pathologically, the lesion in MGUS is in fact very similar to that in multiple myeloma. There is a predominance of clonalplasma cells in the bone marrow with an abnormal immunophenotype (CD38+ CD56+ CD19−) mixed in with cells of a normal phenotype (CD38+ CD56− CD19+); in MGUS, on average more than 3% of the clonal plasma cells have the normal phenotype, whereas in multiple myeloma, less than 3% of the cells have the normal phenotype. What causes MGUS to transform into multiple myeloma is as yet unknown.
At the Mayo Clinic, MGUS transformed into multiple myeloma or similar lymphoproliferative disorder at the rate of about 1-2% a year, or 17%, 34%, and 39% at 10, 20, and 25 years, respectively, of follow-up—among surviving patients. However, because they were elderly, most patients with MGUS died of something else and did not go on to develop multiple myeloma. When this was taken into account, only 11.2% developed lymphoproliferative disorders.
Kyle studied the prevalence of myeloma in the population as a whole (not clinic patients) in Olmsted County, Minnesota. They found that the prevalence of MGUS was 3.2% in people above 50, with a slight male predominance (4.0% vs. 2.7%). Prevalence increased with age: of people over 70 up to 5.3% had MGUS, while in the over-85 age group the prevalence was 7.5%. In the majority of cases (63.5%), the paraprotein level was <1 g/dl, while only a very small group had levels over 2 g/dl. A study of monoclonal protein levels conducted in Ghana showed a prevalence of MGUS of approximately 5.9% in African men over the age of 50.
The protein electrophoresis test should be repeated annually, and if there is any concern for a rise in the level of monoclonal protein, then prompt referral to a hematologist is required. The hematologist, when first evaluating a case of MGUS, will usually perform a skeletal survey (X-rays of the proximal skeleton), check the blood for hypercalcemia and deterioration in renal function, check the urine for Bence Jones protein and perform a bone marrow biopsy. If none of these tests are abnormal, a patient with MGUS is followed up once every 6 months to a year with a blood test (serum protein electrophoresis).
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