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Blood Urea Nitrogen assessment for kidney function and hydration status
Normal range: 7-20 mg/dL
BUN (mg/dL) × 0.357 = Urea (mmol/L)
Urea (mmol/L) × 2.8 = BUN (mg/dL)
Low BUN alone is rarely concerning. Usually seen with liver failure (check LFTs, ammonia) or nutritional deficiency.
| Ratio | Interpretation | Common Causes |
|---|---|---|
| <10:1 | Low ratio | Liver disease, low protein diet, rhabdomyolysis |
| 10-20:1 | Normal ratio | Normal kidney function |
| >20:1 | High ratio - Prerenal azotemia | Dehydration, CHF, GI bleeding, high protein |
| >40:1 | Very high ratio | GI bleeding with volume depletion |
Blood Urea Nitrogen measures the amount of nitrogen in blood from urea, a waste product of protein metabolism. The liver converts ammonia to urea, which is filtered by the kidneys.
BUN assesses kidney function, hydration status, and protein metabolism. It is most useful when interpreted alongside creatinine (BUN/Cr ratio) to differentiate prerenal from intrinsic kidney disease.
BUN is affected by many non-renal factors: diet, hydration, GI bleeding, steroids, and liver function. Creatinine is a more specific marker of kidney function, but BUN/Cr ratio provides additional diagnostic information.