Approximately half of SBP patients will have a small M-protein, typically around 5 g/L  and the most common monoclonal protein type is IgG followed by light chain only. More than 40% of SBP patients have an abnormal sFLC ratio with negative serum and urine immunofixation electrophoresis (sIFE and uIFE) . For patients with EMP, sIFE and uIFE are abnormal in less than a quarter of patients .
Presence of an M-protein can be useful for guiding therapy and may provide prognostic value post-treatment. In most patients the monoclonal protein is markedly reduced upon completion of local radiotherapy, but it only disappears entirely in a minority of patients. Persistence of the monoclonal protein during follow up is prognostic of outcome (Section 21.3), and may indicate the presence of a tumour outside the field of radiotherapy .
The potential utility of sFLC analysis has been investigated in a number of studies, the most comprehensive of which was a retrospective study by Dingli et al.  at the Mayo Clinic. Of 116 patients with a serum sample taken at diagnosis and prior to therapy, the κ/λ sFLC ratio was abnormal in 54 (47%); this included 40% of patients who were negative by sIFE. Consistent with these findings, three smaller studies have reported abnormal κ/λ sFLC ratios in 30 - 68% of patients at diagnosis . A number of other studies have also demonstrated the utility of sFLCs for monitoring patients with elevated involved FLC (iFLC) concentrations , particularly those previously classified as having nonsecretory disease .