What causes foamy urine?

Causes of Foamy Urine

Foamy urine is most commonly caused by proteinuria, particularly when protein excretion exceeds 150 mg/day, and requires evaluation to rule out underlying kidney disease. 1

Primary Causes

Proteinuria

• Approximately 22% of patients complaining of foamy urine have overt proteinuria, making this the most significant clinical correlation 1
• Proteinuria is defined as urinary protein excretion exceeding 150 mg/day and can be an early sign of kidney disease 2
• Increased serum creatinine and phosphate levels are statistically significant risk factors for proteinuria in patients with foamy urine 1

Kidney Disease

• Chronic kidney disease (CKD) is diagnosed by persistent elevation of urinary albumin excretion (albuminuria), low estimated glomerular filtration rate (eGFR), or other manifestations of kidney damage 3
• CKD occurs in 20-40% of people with diabetes and can progress to end-stage kidney disease requiring dialysis or transplantation 3
• Diabetic kidney disease typically develops after a diabetes duration of 10 years in type 1 diabetes but may be present at diagnosis of type 2 diabetes 3

Glomerular Disorders

• Glomerular proteinuria (>2 g/day) is the most common pathophysiologic mechanism causing proteinuria and foamy urine 4
• Common glomerular disorders include:
  • Nephrotic syndrome
  • Glomerulonephritis (post-infectious, membranous, membranoproliferative, lupus, IgA)
  • Genetic defects (Alport syndrome, mesangial sclerosis) 2

Tubular Disorders

• Tubular proteinuria occurs when proximal tubular cells cannot properly reabsorb filtered proteins 5
• Dysfunction of tubular reabsorption may result in increased excretion of albumin, leading to foamy urine 5
• Tubular disorders should be considered when evaluating persistent proteinuria 2

Secondary Causes

Benign/Transient Causes

• Fever, intense activity or exercise, dehydration, emotional stress, and acute illness can cause transient proteinuria and foamy urine 4
• Functional proteinuria may occur in disorders with altered renal hemodynamics and usually resolves without progressive renal disease 6
• Orthostatic proteinuria (protein excretion normalizes in recumbent position) is generally a benign condition 6

Urinary Characteristics

• Alkaline, dilute, or concentrated urine can cause falsely positive protein tests 4
• The presence of mucus, semen, or white blood cells can also lead to foamy appearance of urine 4

Evaluation Approach

Initial Assessment

• Urinalysis with microscopic examination to detect red blood cells, white blood cells, and casts 3
• Quantification of proteinuria using spot urine protein-to-creatinine ratio or 24-hour urine collection 3
• Assessment of kidney function with serum creatinine and estimated glomerular filtration rate (eGFR) 3

Risk Stratification

• Patients with diabetes, poor renal function (high creatinine, BUN, low eGFR), increased serum phosphate, and increased serum glucose are at higher risk for significant proteinuria 1
• Presence of significant proteinuria (>1,000 mg/24 hours), red cell casts, or renal insufficiency should prompt evaluation for renal parenchymal disease 3

Specialized Testing

• Evaluation for dysmorphic red blood cells (glomerular bleeding is associated with >80% dysmorphic RBCs) 3
• Renal ultrasound to assess kidney size, echogenicity, and morphology to differentiate acute from chronic kidney disease 3
• Referral to nephrologist for persistent proteinuria >1,000 mg/24 hours or if underlying etiology remains unclear 3

Clinical Pearls and Pitfalls

• Not all foamy urine indicates proteinuria—only about 20-30% of patients with foamy urine have significant proteinuria 1
• Isolated proteinuria may be benign or may indicate serious underlying kidney disease—careful evaluation is necessary to distinguish between these possibilities 6
• Proteinuria is both a marker for progression of chronic kidney disease and increased cardiovascular risk 5
• Patients with microscopic hematuria, negative initial urologic evaluation, and no evidence of glomerular bleeding should still be followed for development of hypertension, renal insufficiency, or proteinuria 3