Urea

Also known as Blood Urea Nitrogen — BUN

Description

Urea is a form of nitrogenous waste formed from ammonia via the urea cycle in the liver. It freely crosses lipid membranes and is filtered into the glomerular filtrate in the renal glomerulus. A proportion of this filtered urea is reabsorbed passively before final excretion and it forms a vital of the concentration gradient needed to prevent passive diuresis.

Urea is principally a product of amino acid deamination in the liver. Urea is primarily excreted by the kidneys and is the most commonly used test of renal function with creatinine. References: NationWide Laboratories

Clinical Significance

An increase in serum concentrations of urea and creatinine is known as azotaemia and it is found almost exclusively in animals with kidney disease.

Blood urea coentration may be elevated independently of creatinine for the following reasons:

• Gastro-intestinal haemorrhage, as with gastro-duodenal ulceration.
• Diet with high protein or purine content, such as liver.
• Increased catabolism of body tissues, particularly muscle, which may occur in pyrexic animals or those with systemic disease.

Reduced urea concentration may occur due to:

• Over-enthusiastic fluid therapy, diluting the urea present in the blood.
• Disease that cause polyuria and/or polydipsia may cause 'medullary washout' of urea. Common examples include pyschogenic polydipsia, diabetes insipidus and Cushing's disease.
• Reduced synthesis with porto-systemic shunts, microvascular dysplasia or liver failure.
• A diet with a low protein content or malnutrition.

Small Animals

Increased urea, renal azotaemia

• Acute renal failure
• Chronic renal failure

Increased urea, prerenal azotaemia

• Dehydration
• Shock
• High protein diet
• Fever
• Reduced cardiac output
• Hyperthyroidism
• Hypoadrenocorticism
• Gastrointestinal haemorrhage

Increased urea, postrenal azotaemia

• Feline urological syndrome
• Bladder rupture
• Calculi
• Neoplasia
• Perineal herniation

Causes of a low urea concentration

• Polydipsia/polyuria (due to causes other than renal disease)
• Hepatic insufficiency, especially portosystemic shunt
• Low protein diet
• Late pregnancy
• Anabolic steroids

Complementary tests

Creatinine and phosphorus. Urine specific gravity, urinalysis, glomerular filtration rate and SDMA.

Equine

Increased urea, prerenal azotaemia

• Dehydration
• Reduced renal perfusion
• Congestive heart failure
• Excessive muscle catabolism
• High protein diet
• Grass sickness

Increased urea, renal azotaemia

• Acute renal failure
• Chronic renal failure

Increased urea, postrenal azotaemia

• Obstruction, ruptured bladder
• Ruptured bladder in neonatal foals
• Low urea concentrations
• Hepatic insufficiency
• Young foals (normal by 60 days)

Complementary tests

Creatinine and phosphorus. Urine specific gravity (<1.020 in a dehydrated or azotaemic horse suggests renal tubular dysfunction). Urine clearance or fractional electrolyte excretion ratios (renal

tubular dysfunction).

Test Codes - Please visit www.nwlabs.co.uk or see our current price list for more information

References

Text referenced 'Nationwide Laboratories' NationWide Laboratories

Small Animals: NationWide Laboratories

Equine: NationWide Laboratories