How Urine Immunofixation is Performed
Urine immunofixation is performed by collecting a 24-hour urine specimen, concentrating an aliquot from this collection (typically by membrane filtration or desiccation), and then subjecting it to electrophoresis followed by immunofixation using antisera against immunoglobulin heavy chains (IgG, IgA, IgM) and light chains (κ and λ). 1
Specimen Collection and Preparation
The procedure begins with a 24-hour urine collection, which is the standard specimen type recommended by consensus guidelines 1. This extended collection period is necessary because:
- Monoclonal proteins may be excreted intermittently
- It allows for accurate quantification of total protein excretion
- It provides sufficient volume for concentration
Concentration Step
The urine must be concentrated before analysis because monoclonal proteins may be present in very low concentrations. Two primary methods are used:
- Membrane filtration (traditional method): Costs approximately $5 per specimen but is widely standardized 2
- Desiccation (alternative method): Provides comparable or superior results at minimal cost and is technically simpler 2
Recent evidence suggests that using antisera specific to free light chains (FLC) during immunofixation increases sensitivity by 18.4% compared to conventional antisera, with lambda light chains showing nearly 3-fold higher incremental detection rates than kappa 3, 2.
The Immunofixation Process
After concentration, the specimen undergoes:
- Electrophoresis: Proteins are separated based on their electrical charge and size
- Immunofixation: Specific antisera (anti-IgG, anti-IgA, anti-IgM, anti-κ, anti-λ) are applied to separate lanes
- Visualization: Monoclonal proteins appear as discrete bands where antibody-antigen complexes form
Clinical Context and Sensitivity
Immunofixation is more sensitive than protein electrophoresis alone and is essential for:
- Identifying and typing monoclonal immunoglobulins 1
- Determining complete response to treatment 1
- Detecting small amounts of monoclonal protein that may be missed by electrophoresis
Important Technical Considerations
The concentration method matters: Some laboratories now use unconcentrated or minimally concentrated urine with high-resolution gel systems, which show 97-98% concordance with traditional concentrated methods while reducing sample rejections due to insufficient volume 4.
Discordance between serum and urine results can occur due to:
- Intra-molecular disulfide bond formation (particularly with IgA)
- Co-migration of different proteins with similar electrophoretic mobility
- Differences in antibody reactivity to free versus bound light chains 5
Critical Pitfall
Do not use urinary free light chain quantitative assays to measure Bence Jones protein—these assays have not been validated for this purpose. Instead, use urine protein electrophoresis for quantification 1. The immunofixation is for identification and typing only, not quantification.
When to Order
Urine immunofixation should be ordered alongside serum immunofixation when evaluating for monoclonal gammopathies, particularly when:
- Serum studies are negative but clinical suspicion remains high
- Monitoring patients with known light chain disease
- Evaluating renal involvement in monoclonal gammopathies 1, 6
The test detects monoclonal proteins in approximately 99% of light-chain cast nephropathy cases and 96-100% of immunoglobulin-related amyloidosis cases 1.