BUN to Creatinine Ratio Calculator
BUN to Creatinine Ratio Calculator
Calculate the BUN:creatinine ratio, a quick pointer toward pre-renal, renal or other causes of an abnormal kidney profile.
BUN to Creatinine Ratio
BUN ÷ creatinineBUN 32 mg/dL, serum creatinine 1.4 mg/dL
Formula
- BUN
- blood urea nitrogen, mg/dL
- creatinine
- serum creatinine, mg/dL
- note
- outside the US, urea is reported in mmol/L and the urea:creatinine ratio uses different reference values — it is not numerically interchangeable with BUN:Cr
Worked example
BUN 32 mg/dL, serum creatinine 1.4 mg/dL
32 ÷ 1.4 = 22.9
Above 20 → raised ratio, consider pre-renal cause or upper GI bleeding
Interpreting the ratio
| Ratio | Pattern | Consider |
|---|---|---|
| < 10 | Low | Low protein intake, liver disease, malnutrition, overhydration, rhabdomyolysis |
| 10 – 20 | Normal | Urea and creatinine rising in proportion, as in most intrinsic renal disease |
| > 20 | Raised | Pre-renal state (volume depletion, heart failure, renal artery stenosis) or upper gastrointestinal bleeding |
Why urea and creatinine drift apart
Urea and creatinine are cleared differently. Creatinine is filtered at a fairly constant rate relative to muscle mass and undergoes only modest tubular handling, while urea is filtered and then substantially reabsorbed in the proximal tubule, and that reabsorption increases whenever tubular flow slows — most notably when renal perfusion falls. A pre-renal state, in which the kidneys are hypoperfused but not yet structurally injured, therefore raises urea more than creatinine, and the ratio climbs above the usual 10 to 20 range.
Upper gastrointestinal bleeding produces the same pattern by a different route: blood in the gut is digested and absorbed as protein, adding a urea load independent of renal perfusion, so a raised ratio in a patient without obvious volume depletion should prompt a check for occult bleeding.
A low ratio points the other way. Reduced protein intake or liver disease both lower urea production, since urea is synthesised from protein breakdown in the liver; malnutrition and overhydration have a similar diluting effect. Rhabdomyolysis can also lower the ratio by raising creatinine disproportionately, from muscle breakdown rather than reduced clearance.
Units are a common source of confusion. This ratio uses BUN and creatinine both in mg/dL. Outside the United States, urea is usually reported in mmol/L rather than BUN in mg/dL, and the urea:creatinine ratio calculated from those units uses different reference ranges — it is not the same number and should not be read against the 10–20–above-20 thresholds used here.
Frequently asked questions
What does a high BUN:creatinine ratio mean?
Above 20 suggests a pre-renal state — reduced renal perfusion from volume depletion, heart failure or renal artery stenosis — or upper gastrointestinal bleeding, where absorbed blood raises urea disproportionately.
What does a low BUN:creatinine ratio mean?
Below 10 suggests low protein intake, liver disease, malnutrition or overhydration, or a disproportionate rise in creatinine such as rhabdomyolysis.
Is BUN:creatinine ratio the same as urea:creatinine ratio?
No. This calculator uses BUN and creatinine in mg/dL. Outside the US, urea is reported in mmol/L, and the urea:creatinine ratio calculated from SI units uses different reference values entirely.
Can the BUN:creatinine ratio diagnose acute kidney injury on its own?
No — it is a pointer, not a diagnostic test. Interpret it alongside volume status, urine sodium or urea studies, and the trend in both values over time.
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References
- Dossetor JB. Creatininemia versus uremia: the relative significance of blood urea nitrogen and serum creatinine concentrations in azotemia. Ann Intern Med. 1966;65(6):1287–99.
- KDIGO Clinical Practice Guideline for Acute Kidney Injury. Kidney Int Suppl. 2012;2(1):1–138.