Urea
Also known as: blood urea, BUN, blood urea nitrogen, serum urea
The nitrogen-containing waste the liver makes from protein breakdown, cleared by the kidneys. Reported as urea in the UK and as blood urea nitrogen (BUN) in the United States, it rises with kidney impairment but also with dehydration, bleeding into the gut and a high-protein diet.
- Also known as
- blood urea, BUN, blood urea nitrogen, serum urea
- Units
- mmol/L (urea, UK) or mg/dL (BUN, US); urea 1 mmol/L ≈ BUN 2.8 mg/dL
- Measured by
- Kidney function tests
What it is and what it does
Ammonia released when amino acids are broken down is toxic, so the liver converts it into urea, which the kidneys filter; unlike creatinine, a good deal of urea is reabsorbed in the tubules, and more is reabsorbed when the body is dry. Urea therefore rises out of proportion to creatinine in dehydration, heart failure with poor kidney perfusion and gastrointestinal bleeding (digested blood is a protein load), and it falls in liver failure and malnutrition. Read alongside creatinine, the urea-to-creatinine pattern helps sort out whether a kidney problem is pre-renal (poor blood supply) or intrinsic. Very high urea in advanced kidney failure contributes to the nausea, itching and drowsiness of uraemia.
Why it may be higher
- Kidney impairment, acute or chronic
- Dehydration, heart failure, shock (reduced kidney perfusion)
- Gastrointestinal bleeding, high-protein diet, corticosteroids, tissue breakdown
- Urinary obstruction
Why it may be lower
- Severe liver disease (less urea made)
- Malnutrition, low-protein diet, pregnancy
- Over-hydration; some inherited enzyme defects
A high or low value can be associated with several different situations. The result is interpreted with other results, symptoms, history and clinical findings.
What can affect the result
- Hydration and protein intake
- Corticosteroids and tetracyclines
- Recent gut bleeding