Test Catalog

Test ID: CRBF    
Creatinine, Body Fluid

Useful For Suggests clinical disorders or settings where the test may be helpful

Identifying the presence of urine as a cause for accumulation of fluid in a body compartment


Measuring the ultrafiltration capacity of the peritoneal membrane in patients receiving peritoneal dialysis

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Byproducts of nitrogen metabolism are present in high concentration in urine compared to blood and serve as a surrogate marker for the identification of urine leakage into a body compartment. 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)


Peritoneal, abdominal, pelvic drain fluid:

Trauma as well as abdominal or pelvic surgery can lead to bladder perforation or formation of urinary fistula with excessive accumulation of peritoneal fluid or increased surgical drain output caused by intraperitoneal urinary leakage.(1,2)


Pleural fluid:

Urinoma describes the accumulation of urine in the perirenal and retroperitoneal spaces caused by genitourinary tract injury due to trauma or blockage of the urinary tract due to stones, strictures, tumors, benign prostate hypertrophy, etc.(3) Rarely, this fluid can translocate to the pleural cavity causing pleural effusion via movement of urine through the diaphragm or via lymphatic communication between retroperitoneal and pleural spaces caused by increased pressure due to urinoma. Urinothorax is the term used to describe an accumulation of urine in the pleural space. Patients often develop symptoms of dyspnea, chest pain, abdominal pain, and reduced diuresis.(4) The condition is reversed when treatment is directed to correct the primary cause (trauma in 75% and obstruction in 24% of cases). The pleural fluid to serum creatinine ratio is above 1 in 97.9% of cases (n=48; median ratio=2.9, range=0.95-16).


Peritoneal dialysis fluid:

Peritoneal dialysis (PD) is a type of ambulatory dialysis in which hyperosmotic fluid is infused into the patient's peritoneal cavity, with the peritoneum employed as the dialysis membrane promoting the diffusion of small molecules and free water from circulation.(5) The peritoneal equilibration test estimates the rate of small solute transport across the peritoneal membrane and the ultrafiltration capacity. Several analytes may be measured in order to perform this test. Creatinine is measured in PD fluid as well as in plasma or serum in samples taken 2 and/or 4 hours after the dialysate is instilled. The dialysate fluid to serum or plasma creatinine ratio is calculated with larger ratios (approaching 1.0) observed in patients exhibiting faster transport rates.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

An interpretive report will be provided.

Interpretation Provides information to assist in interpretation of the test results

Peritoneal, pleural, and drain fluid concentrations should be compared to serum or plasma. Fluid to serum ratios above 1.0 suggest the specimen may be contaminated with urine.(1-4)


Peritoneal dialysate fluid to serum creatinine ratios can be calculated from timed collections to determine peritoneal membrane transport rates.(5)


All other fluids: results should be interpreted in conjunction with serum creatinine and other clinical findings.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

The manufacturer lists the following interfering substances that affect creatinine results for serum testing and are expected to impact body fluid results in a similar manner, when present:

-Rifampicin, levodopa, and calcium dobesilate (eg, Dexium) cause artificially low creatinine results.

-As tested according to CLSI recommendation methyldopa causes artificially low creatinine results.

-Dicynone (Etamsylate) at therapeutic concentrations may lead to falsely low results.

-N-ethylglycine at therapeutic concentrations and DL-proline at concentrations of 1 mmol/L or greater (> or =115 mg/L) give falsely high results.

-2-Phenyl-1,3-indandion (Phenindion) at therapeutic concentrations interferes with the assay.

-In patients receiving catecholamines (dopamine, dobutamine, epinephrine, and norepinephrine) falsely low results might be observed.


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.


In very rare cases, gammopathy, in particular type IgM (Waldenstrom macroglobulinemia), may cause unreliable results.

Clinical Reference Recommendations for in-depth reading of a clinical nature

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. Austin A, Jogani SN, Brasher PB, et al: The urinothorax: A comprehensive review with case series. Am J Med Sci. 2017;354(1):44–53

4. Toubes ME, Lama A, Ferreiro L, et al: Urinothorax: a systematic review. J Thorac Dis. 2017;9(5):1209-1218

5. Block DR, Florkowski CM: Body fluids. In: Rafai N, Horvath AR, Wittwer CT. eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. Elsevier;2018chap 43