Serum vs Urine Immunofixation Sensitivity
Both serum and urine immunofixation should be performed together, as they are complementary tests with similar sensitivity—immunofixation is more sensitive than protein electrophoresis for both specimens, and performing both serum and urine immunofixation maximizes detection of monoclonal proteins in plasma cell dyscrasias. 1
Comparative Sensitivity
Serum Immunofixation
- Serum immunofixation detects monoclonal proteins in approximately 93.5% of patients with plasma cell dyscrasias who have urinary monoclonal proteins 2
- When combined with serum free light chain assay, serum testing alone missed only 0.5% (2 of 428) of monoclonal gammopathies that had urinary monoclonal proteins, and neither case required medical intervention 2
- Serum immunofixation is essential for identifying and typing monoclonal immunoglobulins and determining complete response 1
Urine Immunofixation
- Urine immunofixation from a concentrated 24-hour collection is required to confirm the presence and type of heavy and light chains, even when there is no measurable protein or peak on urine electrophoresis 1
- Modified urine immunofixation methods using antisera to free light chains demonstrate 18.4% additional positive results compared to conventional methods, with nearly 3-fold higher detection rates for lambda light chains 3
- Urine immunofixation has 74% sensitivity and 100% specificity for identifying patients with renal disease due to monoclonal gammopathies 4
Clinical Algorithm for Testing
Initial Screening
- Perform both serum protein electrophoresis with immunofixation AND 24-hour urine collection with protein electrophoresis and immunofixation simultaneously 1
- Add serum free light chain assay to detect unbound free light chains and determine κ:λ ratio 1, 5
Special Considerations
- Immunofixation should be performed on urine even if there is no measurable protein and even if there is no peak on urine electrophoresis, as light chain-only disorders may be missed 1
- In light chain deposition disease, urine immunofixation is more sensitive than traditional serum or urine electrophoresis methods 4
- For monitoring minimal residual disease, free light chain urine immunofixation appears sensitive enough to serve as a marker of minimal residual disease in multiple myeloma 3
Key Pitfalls to Avoid
Collection Issues
- A 24-hour urine collection cannot be replaced by a random morning urine sample—random samples with creatinine correction require further validation and cannot be recommended 1
- Ensure adequate concentration of the 24-hour urine specimen before immunofixation 1
Monitoring Considerations
- During follow-up of patients undergoing autologous stem cell transplantation or treatment with biological drugs, serum free light chains may detect relapse 4-5 months earlier than serum or urine immunofixation in cases of "light chain escape" 6
- All measurable parameters must be followed throughout monitoring, including both light and heavy chain analysis 7
Bottom Line
The question of which test is "most sensitive" is clinically moot—both serum and urine immunofixation are necessary and complementary. 1 While serum immunofixation combined with serum free light chains detects the vast majority of cases (>99%), urine immunofixation remains essential for complete characterization, quantification of urinary protein excretion (which has prognostic implications), and detection of light chain-only disorders that may be missed by serum testing alone. 1, 2, 3