Paraproteins are detectable by MALDI-TOF MS in the sera of the majority of patients with NSMM and the unique m/z can be used to track the paraprotein through treatment. (A) A 0.086 g/L IgA κ paraprotein that was detectable in the presentation sample of a patient with NSMM (immunofixation negative, IgG 7.31 g/L, IgA 4.1 g/L, IgM 0.8 g/L, serum free κ 79.4 mg/L, serum free λ 22.7 mg/L, sFLCr 3.50) at m/z 11 670 and monoclonal κ FLC were detectable at m/z 11 669 for the doubly charged light chains, respectively. At day+100 after ASCT, the sFLCr had normalized (serum free κ 22.7 mg/L, serum free λ 14.12 mg/L, sFLCr 1.60) but MS detected residual monoclonal κ FLC with the same m/z as detected in the baseline sample. (B) A 0.14 g/L IgG λ monoclonal protein detectable by MS at m/z 11 432 for the doubly charged light chain in the presentation sample from a patient with NSMM (immunofixation negative, IgG 5.40 g/L, IgA 0.5 g/L, IgM 0.39 g/L, serum free κ 5.46 mg/L, serum free λ 6.08 mg/L, sFLCr 0.90). At day+100 after ASCT oligoclonal peaks are present in the IgG and λ spectra but no residual monoclonal protein is detectable at m/z 11 432.