Diagnostic performances of M-protein tests according to the clinical presentations of kidney disease

• Sensitivity of M-protein tests differed by the presenting features of kidney disease.
• M-protein tests showed lower sensitivity in subjects with nephrotic syndrome.
• Serum IF and the FLC ratio is sufficient as screening in subjects with AUA or CGN.
• Urine IF can be added for patients who presented with nephrotic syndrome.

BackgroundScreening for monoclonal immunoglobulin (MIg) is critical in patients with kidney disease.MethodsWe identified 943 subjects who underwent kidney biopsy and at least one of monoclonal (M)-protein tests (serum and urine electrophoresis [EP], serum and urine immunofixation [IF], and serum free light chain [FLC] ratio). The sensitivities of several combinations of the 5 tests were examined by clinical presentations of kidney disease.ResultsThe sensitivities of serum EP, urine EP, and the serum FLC ratio were 65%, 68%, and 71%, respectively, which were lower than those of serum IF (79%) and urine IF (87%) to detect MIg. In the nephrotic syndrome (NS) group, the panel including serum IF, urine IF, and the serum FLC ratio exhibited 100% sensitivity to identify MIg in patients with multiple myeloma (MM) or with monoclonal gammopathy of renal significance (MGRS). In subjects without NS, the panel of serum EP and serum FLC ratio detected MIg in all cases of MM, and the serum IF plus serum FLC ratio detected MIg in all cases of MGRS.ConclusionThis study demonstrated that the sensitivity of screening panels differed by the presenting features of kidney disease. The M-protein tests had lower sensitivity for detection of MIg in subjects with NS compared to those with other clinical presentation.