How is monoclonal gammopathy diagnosed?

How to Diagnose Monoclonal Gammopathy

The diagnosis of monoclonal gammopathy requires three essential tests performed simultaneously: serum protein electrophoresis (SPEP), serum immunofixation electrophoresis (SIFE), and serum free light chain (FLC) assay, with 24-hour urine protein electrophoresis and urine immunofixation as complementary studies. 1

Initial Diagnostic Panel

The optimal diagnostic approach includes the following tests ordered together:

• Serum Protein Electrophoresis (SPEP) serves as the quantitative first-line test that identifies monoclonal protein as a homogeneous spike-like peak in the gamma-globulin zone 1, 2
• Serum Immunofixation Electrophoresis (SIFE) is more sensitive than SPEP and must be performed even when SPEP is negative if clinical suspicion exists, as it confirms monoclonality and identifies the specific immunoglobulin type 1, 2
• Serum Free Light Chain (FLC) Assay measures κ and λ free light chains independently and determines the κ:λ ratio; an abnormal ratio (>1.65 indicates κ clone, <0.26 indicates λ clone) confirms clonality 1, 3
• 24-hour Urine Protein Electrophoresis (UPEP) and Urine Immunofixation (UIFE) detect Bence Jones proteinuria and should be obtained for every patient with serum M-protein >1.5 g/dL or when plasma cell disorders are suspected 1, 2

Why All Three Serum Tests Are Mandatory

Never rely on SPEP alone—this is the most critical pitfall to avoid. The combination approach is essential because:

• SPEP alone misses up to 25% of MGUS cases and 60% of light chain myeloma cases 3
• Light chain MGUS patients have abnormal FLC ratios without detectable M-proteins on SPEP 1
• Approximately 3% of plasma cell disorders are non-secretory with neither serum nor urine proteins detectable by conventional electrophoresis 1
• The FLC assay significantly improves detection when used alongside SPEP and SIFE, particularly for light chain disorders 1

Confirming the Diagnosis

Once a monoclonal protein is detected, the following confirms monoclonal gammopathy:

• Clonality confirmation: Detection of a single immunoglobulin type (e.g., only kappa or only lambda light chains) on immunofixation, as normal plasma cells produce both in balanced ratios 3
• Quantitative immunoglobulins (IgG, IgA, IgM) identify the heavy chain type and quantify the M-protein 3
• Abnormal κ:λ FLC ratio provides independent confirmation of clonality (normal range 0.26-1.65, but rises to 0.34-3.10 in severe renal impairment) 1

When to Proceed to Bone Marrow Examination

• Not routinely recommended for asymptomatic patients with IgG MGUS if serum M-protein ≤15 g/L and no end-organ damage 1
• Required when multiple myeloma is suspected; diagnosis confirmed when >10% clonal plasma cells detected 1
• Mandatory for IgA and IgM M-proteins regardless of concentration as part of the diagnostic workup 1
• Indicated when assessing for smoldering myeloma (M-protein ≥30 g/L or bone marrow plasma cells 10-60%) or active myeloma 3

Specialized Testing for Renal Involvement

When monoclonal gammopathy of renal significance (MGRS) is suspected:

• Kidney biopsy with immunofluorescence showing light chains only, light and heavy chains, or heavy chains only with corresponding serum/urine monoclonal immunoglobulin confirms MGRS 4
• 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 4
• Biopsy is advised for AKI stage 3, eGFR <60 ml/min/1.73m² with >2 ml/min/1.73m² per year decline, proteinuria with hematuria, or albumin:creatinine ratio >30 mg/mmol 4

Critical Pitfalls to Avoid

• Never use different FLC assays for serial monitoring—results between assays (e.g., N Latex vs. FreeLite) are not mathematically convertible; the same assay must be used throughout 1, 3
• Always account for renal function when interpreting FLC levels and ratios, as kidney impairment significantly affects these values 1
• Do not skip immunofixation even with negative SPEP when clinical suspicion is high 1
• Include FLC assay in initial workup—omitting this test misses light chain disorders entirely 1