What is the significance of complement levels, serum/urine protein electrophoresis (SPEP/UPEP), immunofixation, serum free light chains, Antinuclear Antibody (ANA), Phospholipase A2 Receptor (PLA2R), and Hemoglobin A1c (HbA1c) testing in the workup of nephrotic syndrome?

Diagnostic Workup for Nephrotic Syndrome: Role of Specialized Laboratory Testing

In the workup of nephrotic syndrome, serum/urine protein electrophoresis, immunofixation, serum free light chains, complement levels, ANA, PLA2R antibodies, and HbA1c are essential tests that help identify the underlying etiology and guide treatment decisions. These tests provide critical information for distinguishing between primary and secondary causes of nephrotic syndrome, which directly impacts patient morbidity and mortality.

Monoclonal Gammopathy Evaluation

Serum/Urine Protein Electrophoresis and Immunofixation

• Essential for detecting monoclonal immunoglobulins that may cause monoclonal gammopathy of renal significance (MGRS) 1
• SPEP quantifies serum proteins but has limited sensitivity
• Immunofixation of both serum and concentrated 24-hour urine is more sensitive than electrophoresis alone and necessary for identifying and typing monoclonal immunoglobulins 1
• Critical for diagnosing conditions like light-chain deposition disease, AL amyloidosis, and proliferative glomerulonephritis with monoclonal immunoglobulin deposits (PGNMID) 1

Serum Free Light Chain Assay

• Detects unbound free kappa and lambda light chains independently
• Abnormal kappa:lambda ratio indicates clonality (high ratio suggests kappa clone, low ratio suggests lambda clone) 1
• Important considerations:
  • Renal impairment alters free light chain levels (normal ratio 0.26-1.65 can rise to 0.34-3.10 in severe renal impairment) 1
  • Same assay must be used for monitoring as results between different assays are not interchangeable 2
  • Can detect light chain disorders even when SPEP/UPEP are negative 3

Autoimmune and Complement Evaluation

Complement Levels (C3, C4)

• Low complement levels suggest:
  • Lupus nephritis
  • Membranoproliferative glomerulonephritis (MPGN)
  • C3 glomerulopathy 1, 4
• Normal complement levels with monoclonal gammopathy may still indicate complement dysregulation through alternative pathway activation 5
• Serial complement determinations help monitor disease activity and response to therapy 4

Antinuclear Antibody (ANA)

• Screening test for lupus nephritis and other autoimmune diseases that can cause secondary membranous nephropathy 1
• Should be included in the initial workup of immune complex-mediated glomerulonephritis (ICGN) to evaluate for underlying autoimmune disease 1

Anti-PLA2R Antibodies

• Highly specific for primary membranous nephropathy (70-80% of cases) 1, 6, 7
• Positive anti-PLA2R with nephrotic syndrome may obviate the need for kidney biopsy in certain cases 1
• Levels correlate with disease activity and predict response to therapy:
  • High levels (>50 RU/mL) indicate lower chance of spontaneous remission 1
  • Decreasing levels precede clinical remission by 6+ months 1
  • Disappearance indicates immunological remission 1
• Useful for monitoring disease activity and guiding immunosuppressive therapy decisions 1, 7

Metabolic Evaluation

HbA1c

• Essential for diagnosing diabetic nephropathy, a common cause of nephrotic syndrome
• Important to rule out diabetes as a cause of nephrotic syndrome before diagnosing primary glomerular diseases
• Diabetic nephropathy can coexist with other glomerular diseases, including those associated with monoclonal gammopathies 1

Algorithmic Approach to Testing

1. Initial Testing for All Patients with Nephrotic Syndrome:

   • Complete metabolic panel (including creatinine, albumin)
   • Urinalysis with protein:creatinine ratio
   • HbA1c
   • Complement levels (C3, C4)
   • ANA
   • Hepatitis B and C serology

2. Age-Based Additional Testing:

   • Age <40 years:

     • Anti-PLA2R antibodies if features suggest membranous nephropathy

   • Age ≥40 years or any age with concerning features:

     • Anti-PLA2R antibodies
     • SPEP and UPEP with immunofixation
     • Serum free light chain assay
     • Consider age-appropriate cancer screening

3. Based on Initial Results:

   • If anti-PLA2R positive: Primary membranous nephropathy likely; monitor antibody levels to guide therapy
   • If monoclonal protein detected: Complete hematologic workup including bone marrow examination
   • If low complements: Evaluate for lupus, infection-related GN, or C3 glomerulopathy
   • If HbA1c elevated: Consider diabetic nephropathy but don't exclude other concurrent pathologies

Clinical Pearls and Pitfalls

• Important pitfall: Relying solely on SPEP without immunofixation or free light chain analysis may miss significant monoclonal gammopathies 1
• Caution: Normal complement levels don't exclude complement-mediated diseases, as alternative pathway dysregulation can occur with normal serum complement levels 5
• Key consideration: Interpret serum free light chains in the context of renal function, as both kappa and lambda levels increase in renal impairment 1, 2
• Critical point: In patients ≥50 years with C3 glomerulopathy, always evaluate for monoclonal gammopathy even with normal complement levels 1
• Practice alert: Anti-PLA2R antibody trajectory over time is more clinically meaningful than any single value 1

By systematically applying these specialized tests in the workup of nephrotic syndrome, clinicians can more accurately diagnose the underlying cause, implement appropriate therapy earlier, and potentially improve outcomes related to morbidity, mortality, and quality of life.