Blood Urea Nitrogen
BUN· also: Blood Urea Nitrogen, Urea, Blood Urea
Clinical Overview
BUN measures the amount of nitrogen in urea, a waste product formed in the liver from protein metabolism. It is excreted by the kidneys. BUN is used alongside creatinine to assess kidney function and to distinguish kidney causes of azotemia (elevated waste products) from non-kidney causes.
Why This Test Matters
The BUN:creatinine ratio is a key diagnostic tool. A ratio >20:1 suggests pre-renal causes (dehydration, heart failure, GI bleeding) where urea is elevated disproportionately to creatinine. A ratio of 10–20:1 with elevated creatinine suggests intrinsic kidney disease. BUN alone is highly nonspecific — it is affected by protein intake, liver function, catabolic states, and hydration status.
Reference RangesWHO/IFCC standards
| Age Group | Reference Range | Unit | Notes |
|---|---|---|---|
| Adults (18–64) | 7 – 20 | mg/dL | — |
Also reported in: mmol/L.
What Causes Abnormal Results?
High BUN Causes
- Dehydration and pre-renal azotemia (disproportionate BUN rise)
- Kidney disease (elevated BUN + creatinine)
- High protein diet
- GI bleeding (blood is digested and absorbed as protein, raising BUN)
- Catabolic states (sepsis, corticosteroids, burns)
- Heart failure (reduced renal perfusion)
- Urinary tract obstruction
Low BUN Causes
- Liver failure (urea synthesis reduced)
- Malnutrition or very low protein diet
- Overhydration
- SIADH (dilutional)
Signs & Symptoms to Watch For
How to Prepare for This Test
No special preparation required. Inform your doctor of your dietary protein intake and any GI bleeding symptoms.
Factors That Can Affect Results
- High protein diet (raises BUN)
- GI bleeding (digested blood raises BUN)
- Corticosteroids (increase protein catabolism, raising BUN)
- Liver disease (reduces urea synthesis, lowering BUN)
- Hydration status (dehydration raises, overhydration lowers)
Related Topics
Frequently Asked Questions
What does a high BUN:creatinine ratio mean?
A BUN:creatinine ratio above 20:1 suggests the elevated BUN has a non-kidney explanation. The most common causes are dehydration (reduced kidney blood flow — pre-renal azotemia), upper GI bleeding (blood digested as protein), high protein intake, or corticosteroid use. In true kidney disease, both BUN and creatinine rise proportionally, keeping the ratio in the normal range of 10–20:1.