MGUS (Monoclonal gammopathy of unknown significance), aka benign monoclonal gammopathy )BMG) falls under the category of plasma cell disorders in which an abnormal amount of a single immunoglobulin is present in the serum. This monoclonal spike, or M component, is seen in multiple myeloma, Waldenstrom’s macroglobulinemia, primary amyloidosis and various rare heavy chain diseases. MGUS probably represents the common manifestation of multiple disorders and normal variants but it is considered a distinct entity from malignant disorders like non-plasma cell leukemias and lymphomas which occasionally produce an M-spike.

This disorder is fairly common. MGUS increases with age from about 0.1-1% in adults over 25 to 3% in patients over 70, 10% over 80 and more than 20% in patients older than 90. It is more common in blacks than whites and more common in the United States than in Asia. The incidence of multiple myeloma is about .03% per year in patients over 50. This difference in incidence creates the problem of differentiating the rare cases of MM from the more common MGUS.

Whole lists of disorders are associated with a transient monoclonal gammopathy. These include a host of infections, inflammatory diseases, transplants of inorganic material (silicone, valves) and some malignancies. The malignancies are interesting as the M-spike is actually thought to be a plasma cell mediated immune response to the tumor. This spike is common with some tumors known for producing a vigorous immune response, such as melanoma. In most cases of chronic MGUS there is no known cause but a genetic predisposition is clear.