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BUN:Creatinine Ratio (Urea:Creatinine)

BUN:creatinine ratio (both mg/dL) is normally 10–20. A ratio >20 suggests prerenal azotaemia, upper GI bleeding, high protein intake or catabolism; <10 suggests intrinsic renal disease, liver disease or low protein intake. In SI units (urea mmol/L ÷ creatinine µmol/L × 1000), 20 corresponds to about 81.

BUN / serum urea (mg/dL or mmol/L)
Serum creatinine (mg/dL or µmol/L)

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Interpretation

Result	Meaning
< 10	Intrinsic renal / liver disease / low protein
10–20	Normal
> 20	Prerenal, GI bleed, catabolic

Formula

Ratio = BUN (mg/dL) ÷ creatinine (mg/dL). SI ratio = urea (mmol/L) ÷ creatinine (µmol/L) × 1000; SI ≈ conventional × 4.04.

Clinical pearls

Poor discriminator of prerenal vs intrinsic AKI on its own; in one large cohort half of AKI patients had a ratio >20 regardless of cause.
A ratio >30 in a patient without known AKI raises suspicion of upper GI bleeding.

Examples

bun creatinine ratio bun 40 cr 1.0 urea creatinine ratio urea 20 mmol/L creatinine 120 umol/L bun/cr 15 and 1.5

FAQ

What is a normal BUN to creatinine ratio?

About 10 to 20 when both are measured in mg/dL.

How do I calculate the urea:creatinine ratio in SI units?

Divide urea (mmol/L) by creatinine (µmol/L) and multiply by 1000; a value above about 80–100 corresponds to a conventional ratio above 20.

Evidence

Uchino S, Bellomo R, Goldsmith D. The meaning of the blood urea nitrogen/creatinine ratio in acute kidney injury. Clin Kidney J 2012;5:187–91.

Nephrology & Electrolytes · Reviewed 2026-10-14 · Methodology