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 Notes

Formula

BUN = bun / creatinine

Normal Range

10:1 – 20:1

Clinical Notes

Interpret results together with the patient's clinical presentation.

References

MedCalcHub Clinical References

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.

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 to Use

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

When Not to Use

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

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

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

Evidence

Source: NKF / KDIGO
Reference: KDIGO Clinical Practice Guideline for the Evaluation and Management of CKD.
Reviewed By: MedCalcHub Clinical Team
Version: 2024
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.