Background And Definitions

Presence of small amounts of monoclonal protein in the serum detected by protein electrophoresis, in the absence of clinical or laboratory evidence of multiple myeloma (MM), Waldenstrom macroglobulinemia, light chain amyloidosis, or related disorders, was described over half a century ago. Since Waldenstrom's initial description of "essential hyperglobulinemia," this entity has been described by multiple terms, including idiopathic paraproteinemia and idiopathic, asymptomatic, benign, or nonmyelomatous, monoclonal gammopathy. The common theme to all these early descriptions was the presumed "benign" nature of the abnormality and the relatively stable levels of paraprotein in these patients. However, long-term follow-up of these patients has proven that "benign monoclonal gammopathy" is a misnomer. These patients have a definite risk of progression to MM, macroglob-ulinemia, or amyloidosis, and at the time of diagnosis it is impossible to predict with any degree of confidence as to who will have disease progression. This led to coining of the term monoclonal gammopathy of undetermined significance (MGUS), highlighting the uncertain nature of the abnormality at the time of initial detection.

The hallmark of MGUS is the presence of monoclonal protein (M protein) in individuals with no clinical or laboratory evidence of MM, macroglobuline-mia, light chain amyloidosis, or related plasma cell disorders. It is characterized by M protein less than 3 gm/dL in the serum with an absence or small amounts of M protein in the urine; fewer than 10% clonal plasma cells in the bone marrow; absence of bony lytic lesions, anemia, hypercalcemia, or renal insufficiency related to the paraproteinemia. While some of the abnormalities, such as anemia and renal insufficiency, are common in this patient population, it is imperative that an alternate cause for the abnormality be identified prior to making the diagnosis of MGUS. The stability of the paraprotein over time and lack of development of additional abnormalities are important for the definition of MGUS, though at the time of initial detection of the M protein these factors are indeterminate. Some studies that followed these patients for long term have required demonstrated stability of the M protein for at least 1 year for inclusion in the study.

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