Monoclonal gammopathy of undetermined significance and smoldering multiple myeloma.

Monoclonal gammopathy of undetermined significance (MGUS) is characterized by an M spike less than 3 g/dL and a bone marrow containing fewer than 10% plasma cells without evidence of CRAB (hypercalcemia, renal insufficiency, anemia, or bone lesions). Light chain MGUS has an abnormal free light chain (FLC) ratio, increased level of the involved FLC, no monoclonal heavy chain, and fewer than 10% monoclonal plasma cells in the bone marrow. Smoldering multiple myeloma has an M protein of at least 3 g/dL and/or at least 10% monoclonal plasma cells in the bone marrow without CRAB features.