Monoclonal gammopathy of undetermined significance

Monoclonal gammopathy of undetermined significance (MGUS, unknown or uncertain may be substituted for undetermined) or benign monoclonal gammopathy is a condition in which a low or non-quantifiable level of a monoclonal paraprotein is detected in the blood by means of protein electrophoresis. In addition, some patients develop a polyneuropathy (damage to peripheral nerves) or other problems related to the secreted antibody. MGUS is distinct from multiple myeloma, as described below.

Diagnosis

Patients may be diagnosed with MGUS if they fulfill the following three criteria:^[1]

1. A monoclonal paraprotein band less than 30 g/L (< 3g/dL);
2. Plasma cells less than 10% on bone marrow examination; and
3. No evidence of bone lesions, anemia, hypercalcemia, or renal insufficiency related to the paraprotein.

Differential diagnosis

Several other illnesses can present with a monoclonal gammopathy, and the monoclonal protein may be the first discovery before a formal diagnosis is made:

• Multiple myeloma
• AIDS
• Chronic lymphocytic leukemia
• Non-Hodgkin Lymphoma, particularly Splenic marginal zone lymphoma^[2] and Lymphoplasmocytic lymphoma
• Hepatitis C
• Connective tissue disease such as lupus^[3]
• Immunosuppression following organ tranplantation
• Waldenström macroglobulinemia
• Guillain-Barre syndrome^[4]

 Pathology

Pathologically, the lesion in MGUS is in fact very similar to that in multiple myeloma. There is a predominance of clonal plasma cells in the bone marrow with an abnormal immunophenotype (CD38+ CD56+ CD19−) mixed in with cells of a normal phenotype (CD38+ CD56− CD19+);^[5][6] in MGUS, 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.^[7] What causes MGUS to transform into multiple myeloma is as yet unknown.

Prognosis

MGUS may be considered a pre-malignant condition, given the possibility of transformation into multiple myeloma. However, because the condition tends to occur in the elderly, and because the rate of progression is slow, only a small proportion of people with MGUS go on to develop a haematological malignancy. In patients with MGUS, although the actuarial risk of myeloma at 25 years of follow-up is 30%, the actual risk (when competing causes of death are taken into account) is only 11%.^[8]

The annual risk of progressing to multiple myeloma is around 1–2% a year. Kyle et al studied the prevalence of myeloma in a population-wide cohort 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.^[9]

In addition to multiple myeloma, MGUS may also progress to Waldenström's macroglobulinemia, primary amyloidosis, B-cell lymphoma, or chronic lymphocytic leukemia.