BUN Creatinine Ratio Calculator
The BUN creatinine ratio calculator computes the ratio of blood urea nitrogen (BUN) to serum creatinine — a key diagnostic index used to differentiate the cause of elevated BUN or creatinine in clinical practice. Enter your BUN and creatinine values in either American (mg/dL) or Metric (mmol/L / μmol/L) units and the calculator instantly converts and computes the ratio.
Medical disclaimer: This tool is for informational purposes only and does not replace professional medical evaluation. Always consult a licensed physician before making clinical decisions.
Blood Urea Nitrogen (BUN) vs Creatinine
Both BUN and creatinine are metabolic waste products filtered by the kidneys, but they behave differently in the nephron — the functional unit of the kidney:
- Creatinine is freely filtered at the glomerulus, secreted into the tubular lumen, and does not get reabsorbed. It reflects muscle mass and is a stable marker of glomerular filtration.
- BUN (blood urea nitrogen) is filtered at the glomerulus, but approximately 40–50 % is passively reabsorbed in the proximal tubule. When urine flow decreases (as in dehydration), more urea is reabsorbed, raising the BUN disproportionately compared with creatinine.
This difference in handling explains why the BUN/creatinine ratio rises in prerenal states and falls in intrinsic renal disease.
How to Calculate the BUN Creatinine Ratio
The formula is straightforward:
BUN/Creatinine Ratio = BUN (mg/dL) ÷ Serum Creatinine (mg/dL)
Both values must be expressed in mg/dL for the standard ratio. Our calculator automatically converts metric units:
- BUN conversion: mmol/L × 2.8 = mg/dL
- Creatinine conversion: μmol/L × 0.01131 = mg/dL
Simply enter the values in your preferred unit system, and the result is displayed immediately.
BUN Creatinine Ratio — High and Low Values
| BUN/Creatinine Ratio | Interpretation | Common Causes |
|---|---|---|
| > 20 (High) | Prerenal azotemia | Dehydration, excessive protein intake, GI bleeding, heart failure, burns, septic shock, high-dose corticosteroids |
| 10 – 20 (Normal) | Normal renal function | Healthy kidneys with adequate hydration and normal protein metabolism |
| < 10 (Low) | Intrinsic renal or other cause | Acute tubular necrosis (ATN), glomerulonephritis, SIADH, low-protein diet, severe liver disease, rhabdomyolysis |
Blood Urea Nitrogen — Levels and Definition
Blood urea nitrogen (BUN) measures the amount of nitrogen carried in the blood as urea — the end product of protein catabolism in the liver. The kidneys filter urea from the blood and excrete it in urine.
Normal BUN Values
- American units: 7–20 mg/dL
- Metric units: 2.5–7.1 mmol/L
BUN is elevated in dehydration, high protein intake, GI bleeding, renal failure, and heart failure. It may be low in liver disease, malnutrition, or overhydration.
Creatinine — Levels and Definition
Creatinine is a breakdown product of creatine phosphate in muscle tissue. It is produced at a relatively constant rate proportional to muscle mass, making it a reliable endogenous marker of kidney filtration.
Normal Creatinine Values
- American units: Men 0.7–1.2 mg/dL; Women 0.5–1.0 mg/dL
- Metric units: Men 62–106 μmol/L; Women 44–88 μmol/L
Serum creatinine rises when the kidneys lose approximately 50% of their filtering capacity, so it is a late marker of kidney impairment. For earlier detection of CKD, eGFR (estimated glomerular filtration rate) based on creatinine is preferred.
FAQs
What is a normal BUN creatinine ratio?
A normal BUN/creatinine ratio is generally between 10 and 20. A ratio within this range suggests that both BUN and creatinine are elevated (or normal) proportionally, indicating adequate kidney function.
What does a high BUN creatinine ratio mean?
A ratio above 20 suggests a prerenal cause — the kidneys are intact but not receiving enough blood flow (e.g., dehydration, heart failure) or are exposed to an excess nitrogen load (e.g., GI bleeding, high protein diet). Urea gets reabsorbed more than usual, raising BUN faster than creatinine.
What does a low BUN creatinine ratio mean?
A ratio below 10 typically suggests an intrinsic renal problem such as acute tubular necrosis or glomerulonephritis, or a non-renal cause such as a very low-protein diet or severe liver failure. In these conditions, creatinine rises proportionally more than BUN.
Can I use this calculator in metric units?
Yes. Switch the unit toggle to Metric and enter BUN in mmol/L and creatinine in μmol/L. The calculator automatically converts both values to mg/dL before computing the ratio, and also displays the converted equivalent so you can cross-check.
Does the BUN creatinine ratio replace eGFR?
No. The BUN/creatinine ratio is a diagnostic tool for understanding the cause of azotemia — it helps differentiate prerenal from intrinsic renal disease. The eGFR (estimated glomerular filtration rate) is the standard measure of overall kidney function and CKD staging. Use them together for a complete picture.
Are there other kidney function calculators I should use?
Yes. For a comprehensive renal assessment, consider also:
- eGFR Calculator — estimates glomerular filtration rate from serum creatinine
- Albumin Creatinine Ratio (ACR) — detects early kidney damage via urine albumin
- Urine Output Calculator — assesses fluid balance and oliguria risk
- Kt/V Calculator — measures dialysis adequacy