Relationship between free light chain, response status, and indication for initiating second-line therapy. (A) For patients achieving VGPR or better: median absolute values of free light chain at diagnosis, nadir, and initiation of second-line therapy. (B) For patients not achieving VGPR: median absolute values of free light chain at diagnosis, nadir, and initiation of second-line therapy. (C) For patients achieving VGPR or better: median percent chain of free light chain between initiation of therapy and nadir and initiation of second-line therapy. (D) For patients not achieving VGPR: median percent chain of free light chain between initiation of therapy and nadir and initiation of second-line therapy.