Approach to Proteinuria

Normal Protein Excretion

• Uromodulin (Tamm-Horsfall protein) - the dominant fraction
• Albumin: <30 mg/day (the kidneys filter ~4-8 g albumin/day, with nearly all reabsorbed by megalin/cubilin in the proximal tubule)
• Low-molecular-weight proteins (10-20 mg/day) and secreted proteins like IgA (40-60 mg/day)

Mechanisms / Types of Proteinuria

1. Glomerular Proteinuria

• Predominantly albuminuria (albumin is large, 70 kDa, and anionic)
• Can range from microalbuminuria to nephrotic-range proteinuria (>3.5 g/day)
• Causes: minimal change disease, FSGS, membranous nephropathy, MPGN, IgA nephropathy, lupus nephritis, diabetic nephropathy, Alport syndrome

2. Tubular Proteinuria

• Low-molecular-weight proteins (<25 kDa): β₂-microglobulin, retinol-binding protein, α₁-microglobulin
• Normally filtered freely but reabsorbed in the proximal tubule; appear in urine when tubule is damaged
• Usually <2 g/day; dipstick may be negative (dipstick detects intact albumin only)
• Causes: tubulointerstitial nephritis, heavy-metal poisoning (lead, cadmium), Dent disease, Lowe syndrome, Fanconi syndrome, pyelonephritis, ATN
• Tubular proteinuria alongside glomerular proteinuria is an adverse prognostic sign indicating tubulointerstitial damage

3. Overflow Proteinuria

• Occurs when an excess of low-molecular-weight proteins is produced, overwhelming normal tubular reabsorption
• Dipstick negative (non-albumin proteins); SSA test positive
• Causes:
  • Immunoglobulin light chains (Bence Jones protein) - multiple myeloma
  • Lysozyme - myelomonocytic leukemia
  • Myoglobin - rhabdomyolysis
  • Free hemoglobin - intravascular hemolysis
• Key clue: SSA strongly positive + dipstick negative

4. Postrenal (Post-glomerular) Proteinuria

• From inflammation, infection, irritation, or malignancy of the collecting system (renal pelvis, ureters, bladder, prostate)
• Typically low-level, non-albumin, transient

Quantification of Proteinuria

• False positives: alkaline urine (pH >8), iodinated contrast, cephalosporins, chlorhexidine, concentrated urine
• False negatives: dilute urine, non-albumin proteins (Bence Jones, myoglobin)
• Requires confirmation: a 2+ or greater is highly suggestive; trace/1+ is only predictive if specific gravity <1.025

Clinical Evaluation - Step-by-Step

Step 1: Confirm and Quantitate

• Repeat dipstick to confirm (exclude transient causes: fever, exercise, dehydration, UTI)
• Spot urine PCR or ACR
• Consider 24-h urine only if body habitus is unusual (low or high muscle mass) affecting creatinine generation

Step 2: Categorize by Amount

• Microalbuminuria (ACR 30-300 mg/g): primarily for DM/HTN screening - marker of cardiovascular and renal risk
• Nonnephrotic proteinuria (<3.5 g/day): wide differential; may be glomerular, tubular, overflow, or functional
• Nephrotic-range proteinuria (>3.5 g/day): virtually diagnostic of glomerular disease

Step 3: History and Examination

• DM (duration, control), hypertension, SLE, HIV, hepatitis B/C, amyloidosis, malignancy
• Drug history (NSAIDs, lithium, heavy metals, heroin, penicillamine, gold)
• Family history (Alport syndrome, polycystic kidney disease)
• Symptoms: frothy urine, edema, hematuria, hypertension, oliguria

Step 4: Baseline Investigations

• Urine: dipstick + microscopy (casts? red cells? granular casts?), 24-h protein or spot PCR/ACR, urine electrophoresis (if overflow suspected)
• Blood: serum creatinine/eGFR, electrolytes, albumin, lipid profile, glucose/HbA1c, CBC
• Serologies (if glomerular disease suspected): ANA, anti-dsDNA, ANCA (MPO, PR3), anti-GBM, complement (C3, C4), hepatitis B/C, HIV, SPEP/UPEP (myeloma screen), cryoglobulins, ASO titre
• Imaging: renal ultrasound (size, echogenicity, obstruction)

Step 5: Classify the Proteinuria Pattern

• Proteinuria + hematuria + RBC casts + hypertension → nephritic pattern → glomerulonephritis
• Proteinuria >3.5 g + hypoalbuminemia + edema + hyperlipidemia → nephrotic syndrome
• Proteinuria + reduced GFR → CKD evaluation + consider biopsy
• Proteinuria alone (no hematuria, normal GFR, normal BP) → isolated proteinuria pathway (see flowchart below)

Evaluation Flowchart for Isolated Asymptomatic Proteinuria

When to Biopsy?

Proteinuria in Special Populations

Diabetes

• Screen annually with ACR
• Microalbuminuria (ACR 30-300 mg/g) = moderately increased albuminuria; start ACE-I or ARB
• Macroalbuminuria (ACR >300 mg/g) = severely increased albuminuria; optimize RAAS blockade, BP <130/80

Children

• Most transient when followed over weeks
• Persistent = present in ≥2 of 3 weekly samples
• Orthostatic proteinuria most common benign cause - confirmed by normal first-morning (recumbent) urine PCR
• Nephrotic syndrome in children: first presentation at age 1-10 years often treated empirically with steroids (presumed MCD) without biopsy

Management Principles

1. Treat the underlying cause (immunosuppression for GN, antivirals for HBV/HCV-related nephropathy, treat myeloma, etc.)
2. RAAS blockade is the cornerstone of antiproteinuric therapy:
   • ACE inhibitor or ARB reduces intraglomerular pressure and directly decreases proteinuria
   • Titrate to maximum tolerated dose
   • Reduces TGF-β expression, limiting tubulointerstitial fibrosis
   • Angiotensin II directly reduces nephrin expression in podocytes - blockade partially restores this
   • Target: minimize proteinuria (reduction in proteinuria is itself a therapeutic goal)
3. Blood pressure control: target <130/80 mmHg (lower in heavy proteinuria); additional antihypertensives added to RAAS blockade if needed
4. Sodium restriction: reduces RAAS activation, enhances antiproteinuric effect of RAAS blockers
5. Monitoring: reassess urine protein, GFR, and BP every 6-12 months if stable; increase frequency if declining
6. Nephrology referral: when proteinuria >1-2 g/day, when hematuria or GFR decline is present, when biopsy is being considered, or when etiology is unclear

Key Causes of Pathologic Proteinuria (Summary Table)