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Plasma cell dyscrasias are disorders of the plasma cells. Plasma cell dyscrasias are produced as a result of abnormal proliferation of a monoclonal population of plasma cells that may or may not secrete detectable levels of a monoclonal immunoglobulin or immunoglobulin fragment (paraprotein or M protein). Although the most common plasma cell dyscrasia is monoclonal gammopathy of undetermined significance (MGUS), closely related disorders include multiple myeloma, solitary plasmacytoma of bone, extramedullary plasmacytoma, Waldenström's macroglobulinemia (WM), primary amyloidosis, light chain deposition disease, paraproteinemia, and heavy-chain disease. The spectrum of MGUS, solitary plasmacytoma of bone, and asymptomatic and symptomatic multiple myeloma may actually represent a natural progression of the same disease.