BUN/Creatinine Ratio

Q: What does a low BUN/Creatinine ratio mean?

A ratio <10:1 may indicate intrinsic renal disease, liver disease, malnutrition, or a low-protein diet.

Calculates the Blood Urea Nitrogen to Creatinine ratio to help differentiate causes of kidney dysfunction.

Nephrology Internal MedicineEvidence-BasedUpdated 2026-08-06

Clinical decision-support tool for qualified healthcare professionals. Results are not a diagnosis and should not replace professional clinical judgment.

Clinical Notes

The blood urea nitrogen (BUN) to creatinine ratio relates two renally cleared solutes. Urea is produced in the liver, filtered, and substantially reabsorbed by the tubules, whereas creatinine is largely filtered with minimal net reabsorption. Changes in tubular urea handling therefore shift the BUN out of proportion to creatinine. In the appropriate context, an elevated ratio can support the possibility of prerenal physiology, where reduced renal perfusion increases tubular urea reabsorption, while a normal ratio is typical of intrinsic kidney damage. The ratio is supportive rather than diagnostic and should never be used as a definitive threshold for a specific cause. Numerous factors raise BUN independent of renal perfusion, including gastrointestinal bleeding, high protein intake or catabolic states, corticosteroid use, and decreased effective circulating volume from dehydration. BUN can also fall with a low protein diet or liver disease. Any of these can alter the ratio without indicating prerenal azotemia as the mechanism. Many clinical pictures produce an overlap in values, and no single ratio cleanly separates prerenal from intrinsic disease. The ratio is most useful as one element of a broader assessment that includes urine microscopy, the fractional excretion of sodium, urine osmolality, and the patient's volume status and medications. This calculation provides an educational index that supports differential diagnosis of azotemia; it is not a diagnosis of kidney disease and should be interpreted together with concurrent laboratory values and the clinical situation.

Calculator Information

Specialty

Internal Medicine

📋 Reference Ranges

Normal Range

10:1 – 20:1

Reference ranges for this calculator
Classification	Range
Low ratio	<9.1
Normal ratio	10–20
Elevated ratio	≥21

What This Calculator Measures

Calculates the blood urea nitrogen (BUN) to serum creatinine ratio to help differentiate causes of acute kidney injury and assess hydration status.

How to Use

Enter the BUN value in mg/dL.
Enter the serum creatinine value in mg/dL.
Review the ratio and its interpretation.

How the Calculation Works

The BUN-to-creatinine ratio is blood urea nitrogen divided by serum creatinine (BUN / Cr), both in mg/dL. A ratio above about 20:1 suggests a prerenal cause of azotemia.

Variables Explained

Blood Urea Nitrogen

Blood urea nitrogen in mg/dL, used as the numerator.

Serum Creatinine

Serum creatinine in mg/dL, used as the denominator.

Interpretation Guide

A BUN/Cr ratio >20:1 suggests a prerenal cause (dehydration, heart failure, GI bleeding). A ratio of 10–20:1 is normal or suggests intrinsic renal disease. A ratio <10:1 may indicate reduced protein intake, liver disease, or overhydration.

Clinical Significance

The BUN/Cr ratio is a simple, widely available tool for assessing the likely cause of elevated creatinine. It helps guide initial management decisions in acute kidney injury, particularly in distinguishing prerenal from intrinsic renal disease.

When It Is Useful

Evaluation of acute kidney injury
Differentiating prerenal from intrinsic renal causes
Assessment of hydration status
Evaluation of elevated BUN without elevated creatinine

When It Is Not Useful

As a standalone diagnostic tool
In patients on dialysis
When BUN is affected by upper GI bleeding or high-protein diet

Common Mistakes

Entering blood urea nitrogen or creatinine in a unit other than mg/dL, which both inputs require.
Treating a ratio above 20:1 as proof of a prerenal cause instead of one supportive finding.
Interpreting the ratio in a patient on dialysis, where the usual patterns do not apply.

Limitations

Upper GI bleeding, high-protein diet, corticosteroids, and tetracycline can elevate BUN independently of creatinine.
Severe liver disease can lower BUN, reducing the ratio.
Muscle mass affects creatinine, which can alter the ratio in elderly or muscular patients.
Should be interpreted alongside urine electrolytes and clinical context.

Worked Example

A 70-year-old man presents with dehydration. BUN is 45 mg/dL and creatinine is 2.0 mg/dL.

Inputs:

bun: 45

creatinine: 2.0

Expected Result: BUN/Cr ratio = 45/2 = 22.5:1. This elevated ratio suggests a prerenal cause, consistent with dehydration.

Evidence & References

Source:: NKF / KDIGO
Reference:: KDIGO Clinical Practice Guideline for the Evaluation and Management of CKD.
Content updated:: 2026-08

KDIGO Clinical Practice Guideline.
National Kidney Foundation.

References

Powers KS. Acute renal failure: Diagnostic approach and management. Pediatr Rev. 2011;32:109-118. (Review Article)

Frequently Asked Questions

What does a high BUN/Creatinine ratio mean?

A ratio >20:1 may suggest prerenal azotemia (e.g. dehydration, heart failure), GI bleeding, or high protein intake.

What does a low BUN/Creatinine ratio mean?

A ratio <10:1 may indicate intrinsic renal disease, liver disease, malnutrition, or a low-protein diet.

Disclaimer: This calculator is intended for educational and clinical decision support purposes only. The BUN/Cr ratio should be interpreted in the context of the individual patient's clinical presentation and other laboratory values.