Laboratory Tests to Distinguish Between Ascites and Urine

Key Diagnostic Tests

The American Association for the Study of Liver Diseases recommends cell count and differential, total protein, albumin, creatinine, urea, and amylase levels to distinguish between ascites and urine in patients with suspected ascites or urine leakage 1, 2
Cell count and differential can help distinguish between ascites and urine, as ascitic fluid typically contains white blood cells, while urine typically has few or no cells unless there is a urinary tract infection 1, 3
Total protein and albumin levels can also help distinguish between ascites and urine, as ascitic fluid contains protein and albumin at varying levels depending on etiology, while urine typically has minimal protein unless there is kidney disease 1, 4

First-line testing should include obtaining fluid for cell count with differential, total protein, and albumin, as recommended by the American Association for the Study of Liver Diseases 1, 5
Calculating serum-ascites albumin gradient (SAAG) can help confirm ascites due to portal hypertension, with a SAAG ≥1.1 g/dL indicating portal hypertension 1, 6
Measuring creatinine and urea in the fluid can help distinguish between ascites and urine, as significantly elevated levels suggest urine 2

The American Association for the Study of Liver Diseases notes that relying solely on appearance of fluid is a common pitfall, as both ascites and urine can range from clear to cloudy depending on composition and presence of infection 1
Urinary ascites can occur from bladder or ureter injury, resulting in a mixture of characteristics, and creatinine level in the fluid is the most definitive test to distinguish between ascites and urine leakage 2

In cases of trauma or iatrogenic injury, both urine and ascites may be present, requiring comprehensive testing, including cell count and differential, total protein, albumin, creatinine, and urea levels 2
Infected fluid can have elevated white blood cell counts, but ascitic fluid infection typically shows neutrophil predominance above 250/mm³, as noted by the European Association for the Study of the Liver 3