# BUN/Creatinine Ratio Calculator

Calculate your BUN-to-creatinine ratio to help assess kidney function. Distinguish between prerenal azotemia and intrinsic renal disease with clinical interpretation.

## What this calculates

The BUN/Creatinine ratio is a simple blood test calculation that helps clinicians figure out why kidney function markers are elevated. Enter your BUN and serum creatinine values to calculate the ratio and see what range it falls into.

## Inputs

- **Blood Urea Nitrogen (BUN)** (mg/dL) — min 1, max 200
- **Serum Creatinine** (mg/dL) — min 0.1, max 30

## Outputs

- **BUN/Creatinine Ratio** — Ratio of BUN to serum creatinine
- **Interpretation** — formatted as text — Clinical interpretation of the ratio
- **Possible Causes** — formatted as text — Common conditions associated with this ratio range

## Details

Both BUN (blood urea nitrogen) and creatinine are waste products filtered by the kidneys, but they behave differently in various clinical scenarios. That difference is what makes the ratio useful.

**Normal range:** A BUN/Creatinine ratio between 10:1 and 20:1 is considered normal in most adults.

**Elevated ratio (> 20:1):** This pattern often points to prerenal causes, where something is reducing blood flow to the kidneys before they can filter properly. Common scenarios include:

- Dehydration or volume depletion
- Congestive heart failure with low cardiac output
- GI bleeding (digested blood protein raises BUN)
- High protein diet or catabolic states
- Corticosteroid therapy

**Low ratio (< 10:1):** A disproportionately low BUN relative to creatinine suggests:

- Intrinsic kidney damage (acute tubular necrosis)
- Liver disease (reduced urea production)
- Low protein intake or malnutrition
- Rhabdomyolysis (massive creatinine release from muscle breakdown)

The ratio is a screening tool, not a definitive diagnosis. It should always be interpreted alongside the actual BUN and creatinine values, urine output, clinical history, and other labs like urinalysis and fractional excretion of sodium (FeNa).

## Frequently Asked Questions

**Q: What is a normal BUN/Creatinine ratio?**

A: A normal BUN/Creatinine ratio in adults is between 10:1 and 20:1. For example, a BUN of 15 mg/dL and creatinine of 1.0 mg/dL gives a ratio of 15:1, which is normal. Children may have slightly different ranges. The ratio should always be interpreted alongside the absolute values.

**Q: Why does GI bleeding raise the BUN/Creatinine ratio?**

A: When blood enters the GI tract, the proteins in hemoglobin are digested and absorbed as amino acids. The liver converts these amino acids to urea, which raises the BUN. Since creatinine is not affected by GI bleeding, the ratio increases, often above 30:1. An elevated ratio in a patient with suspected GI bleeding supports the diagnosis.

**Q: Can dehydration alone cause an elevated ratio?**

A: Yes. Dehydration reduces blood flow to the kidneys, which causes the kidneys to reabsorb more urea (increasing BUN) while creatinine clearance drops proportionally less. This is classic prerenal azotemia and is usually reversible with fluid resuscitation. Ratios above 20:1 with clinical signs of dehydration strongly suggest a prerenal cause.

**Q: How is this different from the GFR or creatinine clearance?**

A: The GFR and creatinine clearance estimate how well the kidneys are filtering overall. The BUN/Creatinine ratio helps determine why kidney markers are abnormal. A patient with an elevated creatinine might have a GFR of 40, but the BUN/Creatinine ratio helps clarify whether the cause is prerenal (ratio > 20) or intrinsic renal (ratio < 10). They are complementary tools.

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Source: https://vastcalc.com/calculators/health/bun-creatinine-ratio
Category: Health & Fitness
Last updated: 2026-04-08