Serum free light chain (FLC) testing should be performed. This patient meets criteria for a diagnosis of monoclonal gammopathy of undetermined significance (MGUS), because clonal plasma cells represent less than 10% of the total marrow cellularity, and she does not meet CRAB (hyperCalcemia, Renal failure, Anemia, Bone disease) criteria for a diagnosis of multiple myeloma requiring therapy. MGUS is common, affecting 3.2% of persons 50 years or older and 5.3% of persons 70 years or older. The risk of progression to a clinically symptomatic disease is approximately 1% per year. Risk factors for progression include a non-IgG M protein, an M protein level of at least 1.5 g/dL, and an abnormal serum FLC ratio. A few FLCs (not bound to immunoglobulin) circulate normally and can be measured in serum. The FLC ratio measures the κ and λ FLCs, expressed as the κ/λ FLC ratio. Persons without a plasma cell dyscrasia have normal ratios; abnormal ratios suggest a disproportionate production of a monoclonal κ or λ chain. An abnormal FLC ratio is prognostically helpful, because it suggests a more virulent plasma cell clone that may be at higher risk of transforming into overt multiple myeloma. This patient has two risk factors identified thus far (M protein level ≥1.5 g/dL and an IgA M protein). Serum free κ and λ light chain level measurement and κ/λ FLC ratio determination are essential parts of the evaluation of MGUS; they help delineate the risk of progression to clinically symptomatic disease and dictate the frequency of follow-up. Patients with zero, one, two, or three risk factors have a risk of progression to clinically symptomatic disease over 20 years of 5%, 21%, 37%, and 58%, respectively.
MRI imaging of the spine and whole spine MRI have not been shown to further risk stratify patients with MGUS. However, in patients with smoldering (asymptomatic) multiple myeloma, more than one focal bone lesion on MRI is associated with a greater risk of progression to multiple myeloma requiring therapy.
The β2-microglobulin and lactate dehydrogenase levels are useful tools for determining the prognosis of multiple myeloma requiring therapy but are not routinely performed in MGUS.