2. In patients who lack measureable M-protein in serum or urine, i.e., serum M-protein < 0.5 g/dL and urine M-protein < 0.2 g/24 h, serum free light chain (SFLC) levels are most informative. SFLC levels can be used only if the baseline SFLC ratio is abnormal (<0.26 or >1.65), indicating clonality. In addition, the baseline SFLC level must be ≥10 mg/dl of the appropriate involved light chain isotype.