What lab tests can help distinguish between ascites and urine?

Laboratory Tests to Distinguish Between Ascites and Urine

The most effective laboratory tests to distinguish between ascites and urine include ascitic fluid cell count and differential, total protein, albumin, creatinine, urea, and amylase levels. 1

Key Diagnostic Tests

Essential Tests

• Cell count and differential: Ascitic fluid typically contains white blood cells, particularly in the setting of infection, while urine typically has few or no cells unless there is a urinary tract infection 1
• Total protein and albumin: Ascitic fluid contains protein and albumin at varying levels depending on etiology, while urine typically has minimal protein unless there is kidney disease 1, 2
• Creatinine and urea: Significantly elevated in urine compared to ascitic fluid; when these levels in the fluid approach serum levels, this strongly suggests urinary ascites 1

Additional Helpful Tests

• Glucose: Present in ascitic fluid at levels similar to serum; typically absent or minimal in urine unless in diabetic patients 1, 3
• Lactate dehydrogenase (LDH): Can help distinguish ascitic fluid (particularly in secondary peritonitis) from urine 1, 3
• Amylase: Elevated in pancreatic ascites (>1,000 mg/dL) but not typically elevated in urine 1

Diagnostic Algorithm

1. First-line testing:

   • Obtain fluid for cell count with differential, total protein, and albumin 1
   • Calculate serum-ascites albumin gradient (SAAG) if ascites is suspected 1
   • Measure creatinine and urea in the fluid 1

2. Interpretation:

   • If fluid creatinine and urea levels are significantly higher than serum levels, this suggests urine 1
   • If SAAG calculation is possible and ≥1.1 g/dL, this confirms ascites due to portal hypertension 1, 2
   • If cell count shows significant neutrophils (>250/mm³), this suggests infected ascites rather than urine 1

3. Additional testing based on clinical suspicion:

   • If pancreatic ascites is suspected: measure amylase (>1,000 mg/dL suggests pancreatic origin) 1
   • If secondary peritonitis is suspected: measure glucose (<50 mg/dL) and LDH (higher than serum) 1

Clinical Pearls and Pitfalls

• Common pitfall: Relying solely on appearance of fluid. Both ascites and urine can range from clear to cloudy depending on composition and presence of infection 1, 4
• Important consideration: Urinary ascites can occur from bladder or ureter injury, resulting in a mixture of characteristics 1
• Caveat: CA-125 is elevated in patients with ascites of any cause and is not helpful for differential diagnosis 2
• Practical tip: When distinguishing between ascites and urine leakage, the creatinine level in the fluid is the most definitive test - significantly elevated creatinine compared to serum confirms urinary origin 1

Special Situations

• Mixed fluid: In cases of trauma or iatrogenic injury, both urine and ascites may be present, requiring comprehensive testing 1
• Infected fluid: Both infected urine and infected ascites can have elevated white blood cell counts, but ascitic fluid infection typically shows neutrophil predominance above 250/mm³ 1
• Nephrotic syndrome: Can cause ascites with low SAAG (<1.1 g/dL), which may be confused with other conditions 5