Presence of monoclonal free light chains in the serum predicts risk of progression in monoclonal gammopathy of undetermined significance

Summary We hypothesized that the presence of monoclonal free light chains (FLC) in the serum of patients with monoclonal gammopathy of undetermined significance (MGUS) is a marker of clonal evolution and a risk factor for progression. Forty‐seven patients with MGUS and documented progression to myeloma or related malignancy were compared with 50 age‐ and gender‐matched patients with MGUS and no evidence of progression after 5 or more years of follow‐up. The presence of an abnormal kappa/lambda FLC ratio in the serum was associated with a higher risk of MGUS progression (relative risk 2·5; 95% confidence interval: 1·6–4·0; P