Monoclonal gammopathy of undetermined significance (MGUS) is an asymptomatic, premalignant clonal plasma cell proliferative disorder [1–4]. It was initially referred to as essential hyperglobulinemia by Jan Waldenström, as well as several other terms such as benign, idiopathic, asymptomatic, nonmyelomatous, discrete, cryptogenic, and rudimentary monoclonal gammopathy; dysimmunoglobulinemia; lanthanic monoclonal gammopathy; idiopathic paraproteinemia; and asymptomatic paraimmunoglobulinemia [5, 6]. However, since there is an indefinite risk of progression to multiple myeloma (MM) or related disorder such as macroglobulinemia (WM) or amyloidosis (AL), the term MGUS is now the accepted nomenclature [1, 2, 7, 8]. Smoldering multiple myeloma (SMM) is a clinically defined premalignant stage between MGUS and MM [9, 10]. MGUS and SMM must be differentiated from MM, and from a number of related plasma cell disorders using the criteria listed in see Table 1.2 in Chap. 1 [8, 11].
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