BUN/Creatinine Ratio Calculator for AKI Differentiation
The BUN to creatinine ratio is a simple clinical tool used to differentiate between prerenal causes of acute kidney injury (dehydration, volume depletion, CHF, GI bleeding) and intrinsic renal causes.
About
The blood urea nitrogen (BUN) to creatinine ratio has been used for decades as a simple bedside tool to distinguish between prerenal azotemia and intrinsic renal disease. BUN is produced by the liver from protein metabolism and is reabsorbed in the renal tubules, while creatinine is produced from muscle metabolism and is secreted but not reabsorbed. In prerenal states, reduced renal perfusion leads to increased passive reabsorption of urea in the proximal tubule, disproportionately elevating BUN relative to creatinine. In intrinsic renal disease, both BUN and creatinine rise proportionally, maintaining a normal ratio. A ratio >20:1 suggests prerenal causes (dehydration, volume depletion, congestive heart failure, gastrointestinal bleeding, decreased cardiac output). A ratio <10:1 suggests intrinsic renal disease (acute tubular necrosis, glomerulonephritis, interstitial nephritis), liver disease (reduced urea production), low protein intake, or SIADH (dilutional effect). A normal ratio is 10:1 to 20:1. Several factors can confound the ratio: high protein diet, corticosteroids, or GI bleeding can elevate BUN independent of renal function; while liver disease and malnutrition can lower it. The ratio is best interpreted as part of a comprehensive clinical assessment rather than in isolation.
Formula
BUN/Creatinine Ratio = BUN (mg/dL) / Creatinine (mg/dL)
The BUN/creatinine ratio is calculated by dividing the serum BUN concentration by the serum creatinine concentration, both measured in mg/dL. If creatinine is measured in μmol/L, divide by 88.4 to convert to mg/dL before calculating the ratio. The normal ratio is 10:1 to 20:1. A ratio >20:1 suggests prerenal causes, while a ratio <10:1 suggests intrinsic renal or hepatic causes.
Score Interpretation
The BUN/creatinine ratio is a widely used clinical tool in acute kidney injury evaluation, providing rapid differentiation between prerenal and intrinsic renal causes at bedside. It is particularly valuable in the emergency department and inpatient settings where timely diagnosis of prerenal azotemia can prompt appropriate volume resuscitation and prevent progression to intrinsic AKI. The ratio is not diagnostic in isolation and must be interpreted alongside physical examination, urine output, urine sodium, fractional excretion of sodium (FENa), and clinical history.
Normal Ratio — 10–20
BUN/Cr ratio 10-20:1. Normal or intrinsic renal causes.
Management: Interpret in clinical context. Normal ratio does not exclude AKI.
Low Ratio (<10) — 0–9.99
Low ratio. Suggests intrinsic renal disease, low protein intake, liver disease, or SIADH.
Management: Evaluate for intrinsic renal causes. Assess liver function and nutritional status.
Elevated Ratio (>20) — 20.01+
Elevated ratio. Suggests prerenal causes: dehydration, CHF, GI bleeding, or hypovolemia.
Management: Assess volume status. Consider fluid resuscitation if prerenal. Evaluate for GI bleeding.
Reference Ranges
| Population | Normal Range | Notes |
|---|---|---|
| Adults with normal renal function | 10:1 to 20:1 | Ratio may vary with diet, hydration, and medications (steroids increase BUN) |
Dr. Khaled Hassan
Dr. Khaled is a cardiology consultant with experience in acute cardiac care.
View medical review board & editorial policy →Example Calculation
A 78-year-old man presents with confusion, dry mucous membranes, and decreased urine output after 3 days of diarrhea. Labs: BUN 68 mg/dL, creatinine 1.8 mg/dL. Ratio = 68/1.8 = 37.8. This elevated ratio >20:1 suggests prerenal azotemia from volume depletion. The patient receives IV fluids and his renal function improves over 48 hours.
Related Conditions
Related Medications
Common Mistakes
Using the ratio to definitively diagnose AKI etiology
The BUN/creatinine ratio is a supportive tool, not a definitive diagnostic test. It must be combined with history, physical exam, urine studies (FENa, urine osmolality), and response to volume expansion for accurate diagnosis.
Frequently Asked Questions
Can the BUN/creatinine ratio be normal in prerenal AKI?
What medications affect the BUN/creatinine ratio?
References
- Feinfeld DA, Bargouthi H, Nakhoul F, et al. Significance of the BUN/creatinine ratio in the diagnosis and classification of acute renal failure. Clin Nephrol. 2002;58(6):401-405. PubMed
- Uchino S, Bellomo R, Goldsmith D, et al. An assessment of the RIFLE criteria for acute renal failure in hospitalized patients. Crit Care Med. 2006;34(7):1913-1917. PubMed
- KDIGO Clinical Practice Guideline for Acute Kidney Injury. Kidney Int Suppl. 2012;2(1):1-138.