Investigations in Nephrotic Syndrome

1. Urine Investigations

Urine Dipstick

• Detects 3+ or 4+ proteinuria (≥300 mg/dL). Note: dipstick detects only acidic proteins (mainly albumin) and misses light chains (e.g., in myeloma) - these require urine protein electrophoresis (UPEP).

Spot Urine Protein-to-Creatinine Ratio (PCR)

• Correlates well with 24-hour protein excretion; PCR >2.0 mg/mg (or >200 mg/mmol) indicates nephrotic-range proteinuria.
• Corrects for urinary dilution/concentration.
• Nephrotic range in children: protein ≥40 mg/m²/hour or >50 mg/kg/day.

24-Hour Urine Protein

• The gold standard; >3.5 g/24h in adults confirms nephrotic-range proteinuria.

Urine Microscopy

• Oval fat bodies - sloughed tubular cells containing reabsorbed lipid droplets; show "Maltese cross" pattern under polarized light
• Fatty casts - cellular casts containing lipid-laden tubular cells
• Lipid droplets - appear round and clear with a green tinge on light microscopy
• Lipiduria is the fifth feature of nephrotic syndrome; it is a consequence of proteinuria, not of plasma lipid abnormalities
• Microscopic hematuria may be present in ~20% of cases (particularly FSGS, membranous nephropathy, or if there is a nephritic component)
• RBC casts suggest a nephritic overlay

Urine Albumin-to-Creatinine Ratio (ACR)

• More specific for albumin; important when light chain disease is suspected concurrently.

Urine Protein Electrophoresis (UPEP) + Bence Jones Protein

• Detects monoclonal light chains or heavy chains in myeloma-associated disease or amyloidosis.
• Serum free light chain assay may also be used.

2. Blood (Serum) Investigations

Serum Albumin

• <3.5 g/dL (often <2.5 g/dL in florid NS); confirms hypoalbuminemia.
• Rapid-onset hypoalbuminemia may produce Muehrcke's lines (horizontal white bands on nails).
• NS is a hypercatabolic state - proximal tubule catabolism exceeds hepatic albumin synthesis.

Renal Function Tests

• Serum creatinine, urea/BUN, eGFR.
• Usually normal or mildly elevated; significant elevation suggests AKI complication or rapidly progressive course.
• Electrolytes: may show hyponatremia (dilutional).

Lipid Profile

• Total cholesterol, LDL, triglycerides elevated; HDL reduced.
• Elevated LDL is driven by increased hepatic lipoprotein synthesis in response to reduced oncotic pressure and altered lipoprotein metabolism.
• Cholesterol typically >300 mg/dL in florid NS.

Complete Blood Count (CBC)

• Baseline; may reveal haemoconcentration in underfill state or anaemia.
• Elevated fibrinogen contributes to hypercoagulability.

Coagulation Studies

• NS is a hypercoagulable state due to urinary loss of antithrombin III, protein C, protein S, and plasminogen - combined with hyperfibrinogenemia.
• PT, APTT, fibrinogen; D-dimer if thrombosis suspected.
• Membranous nephropathy in particular is most strongly associated with renal vein thrombosis (RVT).

Serum Electrophoresis (SPEP)

• Detects monoclonal paraproteins (myeloma, amyloidosis, LCDD).
• Complement-protein pattern on SPEP may also suggest secondary causes.

3. Investigations to Determine the Underlying Cause

Complement Studies (C3, C4, CH50)

Immunological (Serologic) Panel

• ANA + anti-dsDNA: SLE (lupus nephritis) - also check anti-Sm, anti-C1q
• ANCA (p-ANCA/c-ANCA): vasculitis-associated GN
• Anti-GBM antibody: Goodpasture syndrome
• Anti-PLA2R (anti-phospholipase A2 receptor): found in ~70-80% of primary membranous nephropathy - a non-invasive diagnostic aid, also monitors treatment response and predicts remission
• Anti-THSD7A antibody: another marker for primary MN (PLA2R-negative cases)
• Cryoglobulins + Rheumatoid factor: cryoglobulinemia
• Antistreptolysin O (ASO) / Streptozyme test: poststreptococcal GN

Infectious Serology

• Hepatitis B surface antigen (HBsAg), anti-HBc: HBV-associated membranous nephropathy
• Hepatitis C antibody (anti-HCV) + HCV RNA: MPGN, cryoglobulinemia
• HIV serology: HIVAN (collapsing FSGS)
• Syphilis (RPR/VDRL/TPHA): membranous nephropathy
• Blood cultures: infective endocarditis-related GN
• Malaria serology where endemic

Diabetes Workup

• HbA1c, fasting glucose: diabetic nephropathy is the most common secondary cause in adults.
• Long duration, absence of retinopathy/neuropathy, or atypical presentation warrants biopsy.

Thyroid Function Tests (TFTs)

• Hypothyroidism can present with edema and mimics NS; TFTs baseline.
• Thyroid-binding globulin is also lost in urine.

Tumour Markers / Cancer Screen

• Solid tumours (lung, gastric, intestinal, breast, prostate, bladder) may cause paraneoplastic membranous nephropathy.
• Hodgkin's lymphoma and lymphoproliferative disorders are associated with MCD.
• Targeted workup based on history and examination.

Serum Free Light Chains + UPEP + Bone Marrow (if indicated)

• For suspected amyloidosis (AL or AA), myeloma, or light-chain deposition disease (LCDD).

4. Imaging

Renal Ultrasound

• First-line imaging in all patients with NS.
• Confirms presence of two kidneys; rules out obstruction or anatomic abnormality.
• Kidney size is usually normal in primary NS.
• Large kidneys (>14 cm) are seen in diabetic nephropathy, amyloidosis, HIV nephropathy, or severe acute GN.
• Small kidneys (<9 cm) with cortical thinning suggest advanced CKD - limits biopsy candidacy.
• Doppler ultrasound of renal veins if renal vein thrombosis suspected (sudden flank pain, haematuria, worsening proteinuria, especially in membranous nephropathy).

Chest X-Ray

• Pleural effusions (common in severe NS)
• Pulmonary oedema

CT / MRI / Venogram

• CT pulmonary angiogram or renal vein CT/MRI for suspected thromboembolism (RVT, pulmonary embolism) - particularly relevant in membranous nephropathy.

5. Renal Biopsy

Biopsy Technique Includes Three Modalities:

1. Light microscopy (LM) - identifies the pattern of glomerular injury
2. Immunofluorescence (IF) - detects immune complex deposits (IgG, IgA, IgM, C3, C4, C1q, fibrin)
3. Electron microscopy (EM) - localizes deposits (subepithelial in MN, mesangial in IgAN, subendothelial in MPGN) and identifies foot process effacement (in MCD/FSGS)

6. Summary: Investigations at a Glance

• Comprehensive Clinical Nephrology, 7th Ed. - Laboratory Studies, Imaging & Kidney Biopsy sections
• Harrison's Principles of Internal Medicine, 22E (2025) - Proteinuric States and Nephrotic Syndrome
• Brenner & Rector's The Kidney - Nephrotic Syndrome (multiple chapters)
• ROSEN's Emergency Medicine - Diagnostic Testing in Nephrotic Syndrome
• Smith & Tanagho's General Urology, 19th Ed. - Essentials of Diagnosis
• Campbell-Walsh Urology, Nephrotic Syndrome section