Lab Tests for Evaluating Protein Gap
The most effective lab tests for evaluating a protein gap include serum protein electrophoresis (SPEP), serum free light chain assay, quantitative immunoglobulin levels, and albumin-to-creatinine ratio or protein-to-creatinine ratio in urine samples. 1
Primary Testing Methods
Serum Protein Electrophoresis (SPEP)
- Separates serum proteins into five main fractions:
- Albumin
- Alpha-1
- Alpha-2
- Beta
- Gamma (typically comprises 10-20% of total protein)
- Identifies abnormal patterns in protein distribution
- Detects monoclonal proteins ("M-spike") in the gamma region that may indicate:
- Multiple myeloma
- Monoclonal gammopathy of undetermined significance (MGUS)
- Waldenstrom's macroglobulinemia
- Solitary plasmacytoma
- Amyloidosis 1
Complementary Serum Tests
- Serum immunofixation electrophoresis (SIFE): Identifies specific type of monoclonal protein
- Quantitative immunoglobulin levels (IgG, IgA, IgM): Provides information about specific antibody types
- Serum free light chain (FLC) assay: Essential for detecting light chain disease, especially when SPEP is negative but clinical suspicion remains high 1
Urine Testing Methods
Urine Protein Assessment
- Albumin-to-creatinine ratio (ACR) in untimed urine samples:
- More accurate than protein concentration alone
- Recommended over total protein for evaluation of proteinuria
- More sensitive for detecting low-grade but clinically important albuminuria 2
- Protein-to-creatinine ratio (PCR) in untimed urine samples:
- Useful when albumin is not the primary protein of concern
- More convenient than timed collections 2
- 24-hour urine collection for protein electrophoresis (UPEP) and immunofixation (UIFE):
- Gold standard for quantifying protein excretion
- Recommended when a monoclonal spike is detected on SPEP 1
Interpretation Considerations
SPEP Pattern Analysis
- Monoclonal pattern (sharp, narrow spike): Suggests plasma cell disorder
- Polyclonal increase in gamma region: Indicates reactive or inflammatory process (infection, liver disease, autoimmune disorders)
- Hypogammaglobulinemia: Consider immunodeficiency or protein-losing conditions 1
Factors Affecting Urine Protein Measurements
- Urine concentration affects accuracy of protein/creatinine ratio:
- Dilute urine (specific gravity ≤1.005) may overestimate protein excretion, especially with creatinine ≤38.8 mg/dL
- Concentrated urine (specific gravity ≥1.015) may underestimate protein excretion, especially with creatinine ≥61.5 mg/dL 3
- Dipstick protein measurements have limitations:
- Poor negative predictive value (34%) for ruling out significant proteinuria
- Good positive predictive value (92%) for dipstick values ≥1+ 4
Testing Algorithm
Initial assessment:
- SPEP with quantification of protein fractions
- Serum free light chain assay
- Quantitative immunoglobulin levels
If monoclonal protein detected:
- Serum immunofixation electrophoresis (SIFE)
- 24-hour urine for protein electrophoresis (UPEP) and immunofixation (UIFE)
- Consider bone marrow examination based on clinical suspicion
If polyclonal increase detected:
- Evaluate for underlying inflammatory or infectious conditions
- Consider liver function tests, inflammatory markers
If hypogammaglobulinemia detected:
- Consider immunodeficiency evaluation
- Assess for protein-losing conditions 1
Important Caveats
- Consistent methodology is crucial for serial monitoring
- Nonsecretory disease (approximately 3% of multiple myeloma patients) may have no detectable M-proteins on SPEP
- Small M-proteins may be missed without complementary testing
- Therapeutic monoclonal antibodies can be misinterpreted as disease-related proteins 1
- Urine specific gravity should be considered when interpreting protein/creatinine ratios 3
By following this comprehensive approach to protein gap evaluation, clinicians can accurately diagnose and monitor conditions affecting protein metabolism, with direct impact on patient morbidity and mortality.