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Identifying the presence of urine as a cause for accumulation of fluid in a body compartment
By-products of nitrogen metabolism are elevated in high concentration in urine compared to blood, and serve as a surrogate marker for the identification of urine leakage into a body compartment. This may occur due to trauma, abdominal or pelvic surgery, and bladder perforation. (1,2) Concentrations of creatinine or urea nitrogen that exceed the concentration found in a concurrent sample of blood are suggestive of the presence of urine.(1,3)
Body fluid to serum ratios >1.0 suggest the presence of urine in the sample.
Specimens with total bilirubin values >20 mg/dL may see decreased values. Protein, glucose, ascorbic acid, a-ketoacids, and some cephalosporin antibiotics may interfere with the creatinine assay, causing an overestimation.
Results can be falsely decreased in patients with elevated levels of N-acetyl-p-benzoquinone imine (NAPQI, a metabolite of acetaminophen), N-acetylcysteine (NAC), and Metamizole.
1. Manahan KJ, Fanning J: Peritoneal fluid urea nitrogen and creatinine reference values. Obstet Gynecol 1999;93:780-782
2. Wong MH, Lim SK, Ng KL, Ng KP: Pseudo-acute kidney injury with recurrent ascites due to intraperitoneal urine leakage. Intern Med J 2012;42:848-849
3. Nguyen-Khac E, Thevenot T, Capron D, et al: Are ascitic electrolytes usable in cirrhotic patients? Correlation of sodium, potassium, chloride, urea, and creatinine concentrations in ascitic fluid and blood. Eur J Intern Med 2008;19:613-618