BUN/Creatinine Ratio Calculator
A key diagnostic tool for acute kidney injury
Last updated on: July 6, 2025
BUN/Cr Ratio Calculator
What is the BUN/Creatinine Ratio?
The BUN-to-creatinine ratio is a simple calculation using two common blood test results: Blood Urea Nitrogen (BUN) and serum creatinine. This ratio helps healthcare professionals assess kidney function and hydration status.
It is particularly useful in the differential diagnosis of acute kidney injury (AKI), helping to distinguish between causes that are "before" the kidney (pre-renal), "within" the kidney (intra-renal), or "after" the kidney (post-renal).
Why is the Ratio Important?
- Differentiates Pre-renal AKI: A high ratio (>20) suggests a pre-renal cause like dehydration or decreased blood flow to the kidneys, as urea reabsorption is increased.
- Indicates Intra-renal AKI: A low ratio (<10) suggests damage to the kidneys themselves (e.g., acute tubular necrosis), where urea reabsorption is impaired.
- A normal ratio (10-20) can be seen in both normal patients and those with post-renal obstruction.
Interpreting the Result (Reference Guide)
Ratio < 10:
Suggests intrinsic renal damage (intra-renal AKI), where urea reabsorption is reduced.
Ratio 10 - 20:
This is a normal range, but can also be seen in post-renal obstruction.
Ratio > 20:
Suggests a pre-renal cause, such as dehydration or reduced renal blood flow.
Clinical Context is Key
- High BUN can also be caused by high protein diet, GI bleeding, or catabolic states.
- Low BUN can be seen in liver disease or malnutrition.
- Creatinine levels are influenced by muscle mass.
- Always consult a healthcare professional for an accurate diagnosis.
This page has been medically reviewed by Dr. Khoulah Attia – PharmD, Immunology Specialist.