BUN/Creatinine Ratio
Combine serum blood urea nitrogen and creatinine levels to assess kidney function and differentiate prerenal azotaemia from intrinsic renal causes.
Serum BUN level. Most labs report in mg/dL.
Serum creatinine. Convert automatically between mg/dL and µmol/L.
How to Use This Calculator
Collect same-day labs
Use BUN and creatinine drawn at the same time to avoid misleading ratios from physiologic fluctuations.
Select appropriate units
Enter values in mg/dL, mmol/L (urea nitrogen), or µmol/L (creatinine). The calculator performs the necessary conversions.
Interpret alongside clinical signs
High ratios suggest prerenal states; low ratios may indicate hepatic dysfunction or malnutrition. Always combine with history, volume status, and eGFR.
Formula
BUN/Cr ratio = BUN (mg/dL) ÷ Creatinine (mg/dL)
For SI units: BUN (mmol/L) × 2.801 = mg/dL; Creatinine (µmol/L) ÷ 88.4 = mg/dL.
Normal ratio ≈ 10–20. Values >20 suggest prerenal azotaemia; values <10 suggest low BUN production.
Full Description
The BUN/creatinine ratio contextualises azotaemia by comparing nitrogenous waste production and excretion. Elevated ratios often arise from reduced renal perfusion (e.g., dehydration, heart failure), gastrointestinal bleeding, or high-protein intake. Low ratios point toward hepatic insufficiency, malnutrition, or increased creatinine production. Use the ratio with eGFR, urinalysis, and clinical assessment to identify the origin of kidney dysfunction and guide fluid resuscitation or nephrology referral.
Frequently Asked Questions
Does dialysis affect the ratio?
Yes. Post-dialysis measurements may normalise both BUN and creatinine, limiting ratio utility. Evaluate trends pre- and post-dialysis with nephrology input.
What about high-protein diets?
High protein intake increases BUN, potentially raising the ratio without prerenal azotaemia. Review dietary history and nitrogen balance.
Can medications modify the ratio?
Yes. Corticosteroids, tetracyclines, and gastrointestinal bleeding elevate BUN. Drugs causing rhabdomyolysis increase creatinine, lowering the ratio.
Is the ratio useful in chronic kidney disease?
In advanced CKD the ratio is less specific because both BUN and creatinine rise due to reduced clearance. Evaluate absolute values and eGFR trends instead.