Urea

 
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Physiology

Urea is synthesized by hepatocytes from ammonia generated by catabolism of amino acids derived either from digestion of proteins in the intestines or from endogenous tissue proteins. Urea is excreted by the kidneys, colon (high in horses), saliva and sweat. The rate of urea production in the liver is dependent on hepatic function and digestion and catabolism of protein, i.e. urea formation is decreased in certain liver diseases (e.g. portosystemic shunts, synthetic liver failure, not just hepatic injury) and increased with protein catabolism or protein digestion in the intestine (upper GI bleeding). Urea is excreted by the kidneys, colon (high in horses), saliva and sweat. In ruminants, urea is excreted into the gastrointestinal system (mostly saliva) where it is converted to amino acids and ammonia which are then used for protein production (remember urea is added as a supplement to many bovine diets). Urea is freely filtered through the glomerulus and passively diffuses out of the tubules at a rate dependent on flow rate through the tubules; the remainder of the filtered urea is excreted in urine. At high flow rates, approximately 40% of filtered urea is reabsorbed. At low flow rates, as happens in hypovolemic individuals, approximately 60% of filtered urea is reabsorbed and added back to the blood urea concentration. This explains the high UN levels seen with decreased GFR of any cause.

Measurement of urea concentration in serum is included in chemistry profiles mainly to screen for decreased glomerular filtration rate (GFR). The test for measurement of urea concentration is called urea nitrogen (UN) or serum urea nitrogen (SUN) (blood urea nitrogen [BUN] is not technically correct as UN is not measured in blood); this is where only the concentration of the nitrogen component of urea is measured. In some other countries, the whole urea molecule is assayed. Thus, in the context of this website, “urea” refers to the molecule and “urea nitrogen” the test for measurement of the nitrogen component of urea.

Methodology

Reaction type

Kinetic

Procedure

In the first reaction of this two-step process, the enzyme urease catalyzes the hydrolysis of urea generating ammonium and carbonate ions. In the presence of glutamate dehydrogenase (GLDH) the ammonium ion then reacts with α-ketoglutarate and NADH to form L-glutamate. The oxidation of NADH to NAD+ causes a decrease in absorbance that is measured kinetically, and is proportional to the concentration of urea.

Reaction is shown below:

Urea + H2 urease  > 2NH4+ + CO2

NH4+ + α-ketoglutarate + NADH   GLDH  > L-glutamate + NAD+ + H2O

Units of measurement

Urea and UN concentrations are measured in mg/dL (conventional units) and mmol/L (SI units). The conversion formulas are shown below:

UN [mg/dL] x 0.357= urea [mmol/L]

urea [mg/dL] x 0.1665 = urea [mmol/L]

Sample considerations

Sample type

Serum, plasma, and urine

Anticoagulant

Heparin or EDTA may be used for measuring urea nitrogen in plasma samples. Ammonium heparin should be avoided as it may lead to spuriously high values of urea.

Stability

The stability of urea in serum or plasma samples are as follows: 7 days at 15 – 25 °C or 2 – 8 °C, and 1 year at (-15)-(-25) °C.

The stability of urea in urine is as follows: 2 days at 15 – 25 °C, 7 days at 2 – 8 °C, and 4 weeks at (-15)-(-25) °C. Urine samples should be collected without preservatives.

Interferences

  • Lipemia: Severe lipemia (>1000 lipemia index) may decrease concentrations.
  • Hemolysis: Will increase with severe hemolysis (>1000 hemolysis index).
  • Icterus: Severe icterus may increase  concentrations (>60 icteric index).

Test interpretation

Increased urea concentration

  • Artifact: severe icterus (increased total bilirubin), ammonia contamination (uncommon)
  • Pathophysiologic:
    • Increased protein catabolism: Fever, burns, corticosteroid administration, starvation, exercise.
    • Increased protein digestion: Hemorrhage into the gastrointestinal system, high protein diets.
    • Decreased GFR (azotemia): Due to prerenal, renal or postrenal causes.

Note that urea will be increased with a normal creatinine in the following situations:

    • Increased production of urea, e.g. protein catabolism.
    • Early prerenal azotemia (urea resorption in proximal convoluted tubules is affected by flow rate through the tubules – slowing down of proximal tubular flow rate will increase urea absorption whereas creatinine concentrations are not affected since creatinine is not resorbed in the tubules in most species).

Decreased urea concentration

  • Pathophysiologic:
    • Decreased protein intake or protein anabolism: Dietary restriction of protein, young animals (high anabolic rate).
    • Increased excretion: Any cause of polyuria, e.g. hyperadrenocorticism, diabetes mellitus.
    • Decreased production: Liver disease, enzyme deficiencies in urea cycle.

Discordant urea and creatinine

Urea and creatinine should always be interpreted together and in relation to the glomerular filtration rate. Below is a summary table of interpretations of different urea and creatinine combinations.

i. Interpretation of discordant urea nitrogen and creatinine values
Urea nitrogen Creatinine Interpretation
N / ↓ Early prerenal azotemiaNormal glomerular filtration rate (GFR) with ↑ urea nitrogen
High protein diet, upper gastrointestinal (GI) bleed
↓ GFR with ↓ creatinine
Decreased muscle mass (cachexia)
 N / ↓  ↑ ↓ GFR with ↓ urea nitrogen
Hepatic failure, polyuria-polydipsia (in absence of chronic kidney disease), low protein diet, metabolism of urea nitrogen by GI flora (horses and cattle)
Normal GFR with ↑ creatinine
A normal finding in Greyhounds (increased muscle mass)
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