What is a normal BUN/creatinine ratio?

A normal BUN/creatinine ratio is 10:1 to 20:1. A ratio above 20:1 suggests pre-renal azotemia (dehydration, heart failure, GI bleeding), while a ratio below 10:1 may indicate intrinsic renal disease, liver disease, low protein intake, or rhabdomyolysis.

What does an elevated BUN/creatinine ratio mean?

An elevated BUN/creatinine ratio (>20:1) suggests increased urea production or decreased renal perfusion without proportional creatinine elevation. Common causes include upper GI bleeding, dehydration, high protein diet, catabolic states, and heart failure with prerenal azotemia.

How is the BUN/creatinine ratio used clinically?

The BUN/creatinine ratio helps differentiate prerenal from intrinsic renal causes of acute kidney injury. Combined with FENa, urine osmolality, and clinical context, it guides fluid resuscitation decisions. However, it is influenced by dietary protein, GI bleeding, steroids, and muscle mass.

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BUN/Creatinine Ratio Calculator

Calculate the BUN-to-Creatinine ratio to help differentiate prerenal azotemia from intrinsic renal disease in acute kidney injury.

Inputs

BUN (mg/dL)

Serum Creatinine (mg/dL)

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Enter BUN and Creatinine to calculate.

> 20:1Suggests prerenal azotemia (dehydration, CHF, GI bleed)

10–20:1Normal range

< 10:1Suggests intrinsic renal disease, liver disease, or low-protein diet

Disclaimer: For educational purposes only. Not a substitute for clinical judgment.

When to use

The BUN/Creatinine ratio is a rapid bedside tool used in the workup of acute kidney injury (AKI) to help distinguish prerenal causes from intrinsic renal disease. It is most useful when combined with urinalysis, FENa, and clinical context.

Clinical pearls

A ratio > 20:1 with elevated BUN suggests prerenal azotemia — think dehydration, heart failure, or GI bleeding (blood is digested protein → ↑ BUN).
Steroids and high-protein diets can raise BUN disproportionately, mimicking prerenal azotemia.
A low ratio (< 10:1) may indicate acute tubular necrosis, rhabdomyolysis, liver disease, or malnutrition.
Always interpret alongside FENa, urine osmolality, and clinical presentation.

References

Morgan DB, Carver ME, Payne RB. Plasma creatinine and urea:creatinine ratio in patients with raised plasma urea. BMJ. 1977;2(6092):929-932.
Baum N, Dichoso CC, Carlton CE. Blood urea nitrogen and serum creatinine. Urology. 1975;5(5):583-588.

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About This Tool

What Is the BUN/Creatinine Ratio?

The BUN/Creatinine ratio compares blood urea nitrogen (BUN) to serum creatinine. Both are waste products filtered by the kidneys, but they behave differently in various clinical conditions. BUN is reabsorbed in the proximal tubule (a process enhanced by low tubular flow rates in prerenal states), while creatinine is minimally reabsorbed. This differential handling makes their ratio clinically useful.

How to Interpret

The normal BUN/Cr ratio is approximately 10–20:1. In prerenal azotemia, reduced renal perfusion leads to enhanced BUN reabsorption, elevating the ratio above 20:1. In intrinsic renal disease (ATN), tubular damage impairs BUN reabsorption, dropping the ratio below 10:1. Upper GI bleeding classically causes very high ratios (> 30:1) because digested blood increases the urea load.

🔑 Clinical Pearls

GI bleeding + AKI with BUN/Cr > 30:1 is nearly pathognomonic for an upper GI source (digested blood → urea).
Corticosteroids, tetracyclines, and high-protein feeds can raise BUN out of proportion to creatinine.
In rhabdomyolysis, creatinine rises disproportionately (creatine release from muscle → creatinine), lowering the ratio.
Liver disease reduces urea synthesis, lowering BUN and the ratio regardless of renal function.

Key References

Morgan DB, Carver ME, Payne RB. Plasma creatinine and urea:creatinine ratio in patients with raised plasma urea. BMJ. 1977;2(6092):929-932.
Kellum JA, Lameire N. Diagnosis, evaluation, and management of acute kidney injury: a KDIGO summary. Crit Care. 2013;17(1):204.

Formula last verified: February 2026