Urea

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. 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). 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 gastrointestinal bleeding). 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 methods, the urea molecule is assayed. Thus, in the context of this website, “urea” refers to the molecule and “urea nitrogen” the test for measurement of urea (which is converted to traditional measurements of urea based on the nitrogen content).

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 + H₂O  ^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). Because the method actually measures urea, the manufacturer of most urea reagents convert the provided results into urea nitrogen (mg/dL in conventional units), which is the traditional way of measuring and reporting urea concentrations. Since the nitrogen content of urea (MW 28) is lower than the whole molecule (MW 60), the change in MW between urea and urea nitrogen is 0.467 (28/60). So the UN results (mg/dL) are calculated from the machine provided urea results (mg/dL) as follows: UN = urea x 0.467 in unit equivalent (mg/dL). 

The conversion formula to convert UN in mg/dL to UN in SI units of mmol/L units and vice versa are shown below:

Traditional to SI: UN [mg/dL] x 0.357 = UN [mmol/L]
SI to tradition: UN [mmol/l] x 2.801= UN (mg/dL)

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: May increase with severe hemolysis (>1000 hemolysis index).
• Icterus: Severe icterus may increase concentrations (>60 icteric index).

Test interpretation

Increased urea nitrogen (UN) concentration

• Artifact: Severe icterus (increased total bilirubin concentrations), ammonia contamination (uncommon)
• Physiologic: A mild increase in UN can occur after eating per one study of 100 clinically healthy dogs, but values did not exceed the upper reference limit (stayed within the reference interval) (Yi et al 2022).
• Pathophysiologic:
  • Increased protein catabolism: Fever, severe burns, corticosteroid administration, starvation. Corticosteroid administration does not always increase UN concentrations. In one study in research beagles given 2 mg/kg prednisolone daily for 28 days, the urea nitrogen concentrations did not increase at 28 days versus baseline (Whittemore et al 2019).
  • Increased protein digestion: Hemorrhage into the upper gastrointestinal system and high protein diets can also result in increased UN concentrations. As for the corticosteroid example above, upper gastrointestinal blood loss can, but does not always, increase UN concentrations. In the above-mentioned study, dogs given aspirin had evidence of bleeding from gastric or duodenal ulcers, but the median UN concentrations did not increase compared to baseline and none were above the reference interval (Whittemore et al 2019).
  • Decreased GFR (azotemia): Due to prerenal, renal or postrenal causes. 

Note that UN concentration may be increased with a normal creatinine concentration in the following situations (see table below):

• • Increased production of urea, e.g. protein catabolism, gastrointestinal hemorrhage.
  • 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). This will also happen in an early renal azotemia.

Decreased urea nitrogen 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 nitrogen and creatinine

Urea nitrogen 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.

Interpretation of discordant urea nitrogen and creatinine values

Urea nitrogen	Creatinine	Interpretation
↑	N / ↓	Early prerenal or renal azotemia Renal azotemia in ferrets Normal glomerular filtration rate (GFR) with non-renal causes of ↑ urea nitrogen High protein diet, upper gastrointestinal (GI) bleed, protein catabolism ↓ GFR with non-renal causes of ↓ creatinine Decreased muscle mass (cachexia), sepsis, hyperbilirubinemia (false decrease in creatinine – see creatinine page for degree)
N / ↓	↑	↓ GFR with non-renal causes of ↓ urea nitrogen Hepatic synthetic failure, polyuria-polydipsia (in the absence of chronic kidney disease), low protein diet, metabolism of urea nitrogen by GI flora (horses and cattle) Normal GFR with non-renal causes of ↑ creatinine A normal finding in Greyhounds and other heavy muscled animals (e.g. Draft horses) (increased muscle mass), post-prandial in ferrets