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# Urea and blood urea nitrogen (BUN) as lab tests

Urea is the waste form of protein nitrogen; it rises when the kidney filters less, but also with dehydration or gut bleeding.

Updated 2026-11-01

## Key facts

- It is made in the liver by the urea cycle from ammonium and aspartate, and removed by the kidney.
- BUN = urea × 0.467 (it counts only the molecule's nitrogen); many countries report urea directly.
- It is reabsorbed by the tubule when urine flow is low: dehydration raises BUN more than creatinine.
- High BUN/creatinine ratio (often > 20): prerenal cause, upper GI bleeding, corticosteroids, high-protein diet or catabolism.
- Low BUN: severe liver disease, malnutrition, pregnancy, overhydration, urea cycle defects.

## In detail

- What it measures: the concentration of urea (or its nitrogen) in serum.
- Where it comes from: amino groups pass to glutamate, are released as ammonium and the liver turns them into urea in the cycle that starts in the mitochondrion (CPS-I, OTC) and ends in the cytosol (arginase).
- Why it rises: lower filtration (acute or chronic kidney disease), lower circulating volume (dehydration, heart failure), higher protein load (blood digested in the gut, diet, catabolism, corticosteroids, tetracyclines).
- Pitfalls: BUN depends on protein intake, liver function and hydration, so it is a poorer marker of filtration than creatinine.
- In uraemia (advanced kidney disease) many toxins build up besides urea; BUN serves as a marker of that build-up and of dialysis adequacy.

**Also called**
BUN, Blood urea nitrogen, Serum urea, BUN/creatinine ratio

## Clinical notes

- A BUN rising out of proportion to creatinine in a patient with melaena suggests upper GI bleeding.
- In advanced cirrhosis BUN may be low and ammonia high, because the liver cannot complete the urea cycle.
- Pericarditis, encephalopathy and bleeding from platelet dysfunction are features of severe uraemia.

## Pathways

- [Urea cycle](https://durgical.com/en/biochemistry/pathways/urea-cycle/)

## Related conditions

- [Acute kidney injury](https://durgical.com/en/conditions/acute-kidney-injury/)
- [Chronic kidney disease](https://durgical.com/en/conditions/chronic-kidney-disease/)
- [Upper gastrointestinal bleeding](https://durgical.com/en/conditions/upper-gastrointestinal-bleeding/)
- [Cirrhosis](https://durgical.com/en/conditions/cirrhosis/)
- [Ornithine transcarbamylase (OTC) deficiency](https://durgical.com/en/conditions/ornithine-transcarbamylase-deficiency/)

## Related

- [Urea](https://durgical.com/en/biochemistry/molecules/urea/)
- [Ammonia (ammonium)](https://durgical.com/en/biochemistry/molecules/ammonia/)
- [Creatinine and estimated glomerular filtration rate (eGFR)](https://durgical.com/en/biochemistry/topics/creatinine-and-estimated-glomerular-filtration-rate/)
- [Transamination and deamination](https://durgical.com/en/biochemistry/topics/transamination-and-deamination/)

## Test yourself

Think of the answer, then tap the question to see it.

## Which organ makes urea, and from what?

The liver, through the urea cycle, from ammonium and aspartate derived from amino acids.

## Why does dehydration raise BUN more than creatinine?

With low urine flow the tubule reabsorbs more urea; creatinine is hardly reabsorbed.

## Why does BUN rise in upper GI bleeding?

Blood is digested in the gut as a protein load that the liver turns into urea, and there is also hypovolaemia.

## What does a very low BUN with high ammonia suggest?

Failure of the urea cycle: severe liver disease or an inborn defect such as OTC deficiency.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [BUN (Blood Urea Nitrogen): MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/bun-blood-urea-nitrogen/) (MedlinePlus, US National Library of Medicine, 2026); [Kidney Tests: MedlinePlus](https://medlineplus.gov/kidneytests.html) (MedlinePlus, US National Library of Medicine, 2026); [Ammonia blood test: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003506.htm) (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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