What tests are used to diagnose gammopathy?

Diagnostic Tests for Gammopathy

The diagnostic workup for gammopathy should include serum protein electrophoresis, serum immunofixation, and serum free light chain assay as the primary tests, with 24-hour urine protein electrophoresis and immunofixation as complementary studies. 1

Initial Laboratory Evaluation

• Serum Protein Electrophoresis (SPEP): First-line test that is quantitative, easy to perform, and inexpensive; identifies monoclonal protein as a homogeneous spike-like peak in the gamma-globulin zone 1, 2
• Serum Immunofixation (IFE): More sensitive than SPEP and necessary for identification and typing of monoclonal immunoglobulins; should be performed even if SPEP is negative when clinical suspicion is high 1
• Serum Free Light Chain Assay: Critical test that measures κ and λ free light chains independently and determines the κ:λ ratio; clonality can be inferred from an abnormal ratio (high ratio indicates κ clone, low ratio indicates λ clone) 1
• 24-hour Urine Collection: For protein electrophoresis and immunofixation; provides total protein level, urinary albumin level, and globular protein component 1
• Urine Immunofixation: Should be performed on concentrated urine aliquot from 24-hour collection; more sensitive than urine protein electrophoresis 1, 3

Additional Testing Considerations

• Nephelometric Quantification: For measurement of serum immunoglobulins (IgG, IgA, IgM) 3
• Immunoblotting: Highly sensitive technique that can detect small amounts of monoclonal immunoglobulin and characterize IgG heavy-chain subclasses; not widely available 1
• Bone Marrow Aspirate/Biopsy: Required when multiple myeloma is suspected; diagnosis confirmed when >10% clonal plasma cells are detected 1
• Cytogenetics: Standard metaphase cytogenetics and FISH provide important prognostic information 1

Important Clinical Considerations

• Free Light Chain Interpretation: Renal function affects free light chain concentration; 'normal' κ:λ ratio (0.26-1.65) can rise to 0.34-3.10 in severe renal impairment 1
• Assay Consistency: Different free light chain assays (e.g., N Latex vs. FreeLite) are not mathematically convertible; the same assay must be used throughout patient monitoring 1
• Complementary Testing: When one test is negative but clinical suspicion remains high, consider using alternative tests as they have different performance characteristics 1, 4

Diagnostic Algorithm

1. Initial Screening:

   • SPEP with high-resolution agarose gel 3, 4
   • Serum free light chain assay 1
   • Serum immunofixation 1

2. If Initial Tests Positive:

   • Proceed with 24-hour urine collection for protein electrophoresis and immunofixation 1, 3
   • Consider bone marrow examination if multiple myeloma is suspected 1

3. If Initial Tests Negative but Clinical Suspicion High:

   • Consider alternative free light chain assay 1
   • Perform 24-hour urine collection for protein electrophoresis and immunofixation 1, 5
   • Consider specialized testing such as immunoblotting if available 1

Pitfalls and Caveats

• High-resolution SPEP has higher sensitivity but more false positives requiring follow-up immunofixation 4
• Relying solely on serum studies (SPEP, IFE, free light chain) may miss approximately 0.5% of monoclonal gammopathies detectable only in urine 5
• Small monoclonal proteins may be missed by SPEP alone, necessitating immunofixation 1, 4
• Renal impairment affects free light chain levels and ratios; interpretation must account for kidney function 1