Input Values
Healthy adult with normal kidney function
Why This Health Metric Matters
Why: This calculation helps assess important health parameters for clinical and personal wellness tracking.
How: Enter your values above and the calculator will apply validated formulas to compute your results.
- ●Evidence-based calculations
- ●Used in clinical settings worldwide
- ●Regular monitoring recommended
Sample Scenarios
Lab Values
Patient Information
Clinical Context
⚠️For informational purposes only — not medical advice. Consult a healthcare professional before acting on results.
🏥 Health Facts
— WHO
— CDC
What is BUN/Creatinine Ratio?
The BUN/Creatinine ratio compares blood urea nitrogen to serum creatinine to help differentiate between prerenal, intrinsic renal, and postrenal causes of kidney dysfunction. This simple calculation provides valuable information about the nature of azotemia when kidney function is impaired. Normal ratio is 10-20:1, with elevations suggesting prerenal causes and lower ratios suggesting intrinsic kidney injury.
Quick Assessment
Simple calculation from routine chemistry panels to guide differential diagnosis in AKI workup
Diagnostic Value
Helps distinguish prerenal azotemia from intrinsic renal causes like ATN, guiding treatment approach
Clinical Utility
Guides fluid resuscitation decisions, workup strategy, and helps identify GI bleeding
How the Ratio Works
BUN Physiology
BUN is produced in the liver from protein metabolism and passively reabsorbed in the proximal tubule. Reabsorption increases with slow tubular flow (prerenal states).
Creatinine Physiology
Creatinine is produced at a constant rate from muscle and freely filtered. It is not significantly reabsorbed, making it a better marker of GFR.
Prerenal Pattern
In prerenal states (dehydration, low cardiac output), increased BUN reabsorption leads to disproportionate BUN elevation → High ratio (>20:1).
Intrinsic Renal Pattern
In intrinsic renal disease (ATN), tubular dysfunction impairs BUN reabsorption → Both rise proportionately or low ratio (<10:1).
When to Use BUN/Creatinine Ratio
Acute Kidney Injury
Differentiate prerenal from intrinsic renal causes of AKI
GI Bleeding Workup
Elevated ratio (> 30:1) suggests upper GI source
Volume Assessment
Assess dehydration or hypovolemia status
Heart Failure
Evaluate cardiorenal syndrome and perfusion
Liver Disease
Low ratio may indicate decreased urea synthesis
Fluid Response
Monitor response to IV fluid resuscitation
BUN/Creatinine Ratio Interpretation & Formulas
Primary Formula
BUN/Creatinine Ratio = BUN (mg/dL) ÷ Creatinine (mg/dL)Both values must be in the same units (typically mg/dL in US, mmol/L in international settings). When using SI units, multiply the result by 2.8 to get equivalent ratio.
Unit Conversion
BUN: mg/dL x 0.357 = mmol/L urea | mmol/L urea x 2.8 = mg/dL BUN
Creatinine: mg/dL x 88.4 = umol/L | umol/L x 0.0113 = mg/dL
Reference Ranges
| Ratio | Interpretation | Common Causes |
|---|---|---|
| <10:1 | Low | ATN, liver disease, malnutrition |
| 10-20:1 | Normal | Normal kidney function |
| 20-30:1 | Elevated | Prerenal azotemia, dehydration |
| >30:1 | Markedly Elevated | Severe prerenal, GI bleeding |
Frequently Asked Questions
Why is the ratio elevated in GI bleeding?
Blood in the GI tract is digested and absorbed as protein, leading to increased urea production (approximately 10-20 mg/dL BUN per unit of blood). This protein load raises BUN disproportionately while creatinine remains stable, elevating the ratio often above 30:1.
Can medications affect the ratio?
Yes. Corticosteroids increase protein catabolism, raising BUN. Cimetidine and trimethoprim block creatinine secretion. NSAIDs, ACE inhibitors, ARBs, and diuretics can cause prerenal AKI which elevates the ratio. Tetracyclines have an anti-anabolic effect increasing BUN.
Is the ratio reliable in all patients?
The ratio has limitations in patients with muscle wasting or cachexia (low creatinine production), liver disease or cirrhosis (low BUN production), vegetarian diet, or those on dialysis. Always interpret in clinical context alongside urine studies and other markers.
What if both BUN and creatinine are elevated?
When both are elevated, the ratio helps determine the cause. High ratio (>20:1) suggests prerenal cause even with elevated creatinine. Normal ratio (10-20:1) with elevated values suggests intrinsic renal disease. Low ratio (<10:1) points toward ATN or rhabdomyolysis.
How does protein intake affect the ratio?
High protein diet or parenteral nutrition with amino acids increases BUN production, elevating the ratio. Conversely, severe malnutrition or low protein intake decreases BUN production, potentially lowering the ratio regardless of kidney function.
What other tests help distinguish prerenal from intrinsic AKI?
Fractional excretion of sodium (FeNa) is more reliable: <1% suggests prerenal, >2% suggests ATN. Urine osmolality (>500 mOsm/kg in prerenal), urine sodium (<20 mEq/L in prerenal), and urine microscopy (muddy brown casts in ATN) also help differentiate.