Management of Monoclonal Gammopathy of Undetermined Significance (MGUS)

Epidemiology

The International Myeloma Society recommends serum protein electrophoresis (SPEP), serum immunofixation electrophoresis (SIFE), and serum free light chain (FLC) assay as the initial diagnostic tests for monoclonal gammopathy, with 24-hour urine protein electrophoresis and urine immunofixation as complementary studies 5
The American Society of Clinical Oncology suggests that SPEP alone misses up to 25% of MGUS cases and 60% of light chain myeloma cases, highlighting the importance of a combination approach 6

The presence of a single immunoglobulin type (e.g., only kappa or only lambda light chains) on immunofixation confirms clonality, as normal plasma cells produce both in balanced ratios 6
An abnormal κ:λ FLC ratio provides independent confirmation of clonality, with a normal range of 0.26-1.65, but rises to 0.34-3.10 in severe renal impairment 5

The National Kidney Foundation recommends kidney biopsy with immunofluorescence showing light chains only, light and heavy chains, or heavy chains only with corresponding serum/urine monoclonal immunoglobulin to confirm monoclonal gammopathy of renal significance (MGRS) 7
Laser microdissection followed by mass spectrometry (LC-MS) is the gold standard for amyloid typing, essential for typing renal amyloidosis in 15% of patients where immunofluorescence is insufficient 7

The European Society for Medical Oncology advises against using different FLC assays for serial monitoring, as results between assays are not mathematically convertible; the same assay must be used throughout 5, 6
The International Myeloma Society recommends accounting for renal function when interpreting FLC levels and ratios, as kidney impairment significantly affects these values 5