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.

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.

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.

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.

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

Sources: BUN (Blood Urea Nitrogen): MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2026); Kidney Tests: MedlinePlus (MedlinePlus, US National Library of Medicine, 2026); Ammonia blood test: MedlinePlus Medical Encyclopedia (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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Representative model, for educational purposes only.