BUN/Creatinine Ratio: Dehydration Detector
Content last reviewed 2026-09-12 · Educational information — not medical advice
Quick answer
The BUN:creatinine ratio (>20:1) screams dehydration or GI bleeding; (<10:1) whispers liver trouble. It's medicine's cheapest hydration-status calculator, derived free from any basic metabolic panel.
Why the ratio outperforms either number
BUN fluctuates with hydration/protein/bleeding while creatinine tracks muscle/kidney steadily — their DIVERGENCE localizes causes. Ratio math turns two confusing numbers into one diagnostic sentence.
Ratio interpretation table
| Ratio | Classic meaning | Supporting clues |
|---|---|---|
| >20:1 | prerenal/dehydration, GI bleed, catabolic state | dark urine, thirst, tachycardia; melena history |
| 10–20:1 | normal or balanced intrinsic disease | rises TOGETHER proportionally in kidney injury |
| <10:1 | liver failure, malnutrition, SIADH dilution | low BUN with stable creatinine; ammonia issues clinically |
Practical scenarios
- Grandma confused, BUN 45/creatinine 1.2 (ratio 37): dehydration driving delirium — fluids fix cognition
- Cirrhosis patient, BUN 6/creatinine 1.0 (ratio 6): failing urea cycle — expected pattern
- Bodybuilder, both elevated proportionally: muscle mass + protein shakes — physiologic
Caveats
Age raises baseline BUN modestly; steroids and burns inflate catabolic ratios; tetracycline affects urea generation. Ratios interpret alongside sodium/osmolality and clinical volume exam — arithmetic alone doesn't hydrate anyone.
Order this test online
Direct-access laboratory testing — no appointment or doctor visit required. Results delivered as secure PDFs.
We may earn a commission when you order through partner links. Educational content only — never a substitute for professional medical advice.
Related guides
Frequently asked questions
High-protein diet raising my ratio — harmful?
Physiologic inflation in healthy kidneys — document it so future readers don't chase phantom renal decline.
Why did ER keep calling me 'prerenal'?
Ratio >20 with concentrated urine = kidney-deprivation not kidney-disease — fluid-responsive by definition.