BUN/Creatinine Ratio

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BUN/Creatinine Ratio

Helps differentiate between prerenal and intrinsic causes of kidney dysfunction

All tools /

BUN/Creatinine Ratio

Helps differentiate between prerenal and intrinsic causes of kidney dysfunction

BUN

Creatinine

BUN/Creatinine Ratio — —

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Instructions

To use this tool, enter the blood urea nitrogen (BUN) level and serum creatinine value. The calculator will divide BUN by creatinine to yield the ratio. Ensure values are entered in the same units (mg/dL is standard). The result should be interpreted alongside clinical context, physical examination, and other laboratory data. This tool provides guidance but does not replace medical judgment or diagnostic workup.

Overview

When to use

Why use

Evidences

Interpretation

BUN creatinine ratio = (blood urea nitrogen in mg/dL) / (serum creatinine in mg/dL)

The blood urea nitrogen (BUN)–to–creatinine ratio is a simple index calculated as BUN (mg/dL) divided by serum creatinine (mg/dL), historically used to help differentiate prerenal azotemia from intrinsic renal causes and to contextualize urea handling relative to creatinine

https://www.ncbi.nlm.nih.gov/books/NBK539757/

Large retrospective analyses and ED-focused studies find BCR performs poorly to distinguish prerenal AKI from intrinsic AKI, with substantial overlap and confounding; in a 20,126‑patient hospital cohort, high BCR (>20) did not identify a lower‑risk “prerenal” group and was associated with higher hospital mortality, contradicting classic teaching

https://pmc.ncbi.nlm.nih.gov/articles/PMC5783213/

The largest ED study specifically assessing diagnostic performance concluded BCR had no discriminative ability to separate prerenal from intrinsic AKI at any threshold, citing multiple physiologic and clinical confounders that alter urea independent of perfusion

https://pmc.ncbi.nlm.nih.gov/articles/PMC5445342/

Upper GI bleeding increases urea absorption and can markedly elevate BUN and BCR; observational work suggests ratios above roughly 23–35 may help suggest an upper GI source when the bleeding site is uncertain, though thresholds vary by study and population

https://pmc.ncbi.nlm.nih.gov/articles/PMC6637801/

Biomarker reviews emphasize that both BUN and creatinine are influenced by many nonrenal factors, limiting their sensitivity and specificity for AKI cause; comprehensive assessment (history, exam, urine studies, imaging) remains essential

https://pmc.ncbi.nlm.nih.gov/articles/PMC3287955/

Overview

When to use

Why use

Evidences

The BUN/Creatinine Ratio is a commonly used clinical parameter to evaluate renal function and to help distinguish prerenal azotemia from intrinsic renal disease. Blood urea nitrogen (BUN) reflects protein metabolism, while serum creatinine is a marker of muscle metabolism. Since both are excreted by the kidneys, their ratio can highlight imbalances caused by dehydration, impaired perfusion, or kidney pathology.

A high ratio (>20:1) usually suggests prerenal causes such as volume depletion or decreased renal perfusion. In contrast, a low or normal ratio may suggest intrinsic kidney disease or liver dysfunction. This tool is useful in both inpatient and outpatient settings, especially for patients presenting with elevated creatinine or urea levels, unexplained azotemia, or suspected acute kidney injury.

The ratio is simple, quick, and requires no additional testing beyond routine blood work, making it a widely adopted index in clinical practice. However, interpretation must consider factors such as dietary protein intake, medications, gastrointestinal bleeding, and comorbidities that affect urea or creatinine metabolism.