BUN Ratio Calculator — quick BUN to Creatinine ratio

Calculate your BUN/Creatinine ratio instantly with clinical interpretation. Simply enter your lab values below.

BUN Ratio
14.0
Low (<10) Normal (10-20) High (>20)

Interpretation

Disclaimer: This calculator is for informational purposes only and is not a substitute for professional medical advice. Always consult a healthcare provider for diagnosis and treatment.

How to use the BUN Ratio Calculator

Using our BUN ratio calculator is simple:

  1. Enter your BUN (Blood Urea Nitrogen) value from your lab report
  2. Select the unit for your BUN value (mg/dL or mmol/L)
  3. Enter your serum creatinine value
  4. Select the unit for your creatinine value (mg/dL, µmol/L, or mmol/L)
  5. Click “Calculate” to get your BUN ratio and interpretation

Example: BUN 14 mg/dL ÷ Creatinine 1.0 mg/dL = 14 (normal)

BUN Ratio result and interpretation

The BUN to creatinine ratio (BUN:Cr) is calculated by dividing the blood urea nitrogen (BUN) value by the serum creatinine value. This ratio helps clinicians distinguish between different causes of kidney dysfunction.

What a high BUN:Creatinine ratio means

A high BUN ratio (>20:1) typically suggests conditions that decrease blood flow to the kidneys (pre-renal causes) rather than intrinsic kidney damage. Common causes include:

  • Dehydration or volume depletion
  • Heart failure
  • Gastrointestinal bleeding
  • High-protein diet
  • Certain medications (corticosteroids, tetracyclines)
  • Urinary tract obstruction

What a low BUN:Creatinine ratio means

A low BUN ratio (<10:1) often indicates conditions affecting the kidneys themselves (intrinsic renal causes). Possible causes include:

  • Acute tubular necrosis
  • Chronic kidney disease
  • Liver disease (impaired urea synthesis)
  • Malnutrition or low protein intake
  • Rhabdomyolysis (muscle breakdown)
  • Sickle cell disease

Causes & clinical context

The BUN creatinine ratio provides valuable clinical information, but it must be interpreted in the context of the patient’s overall clinical picture. Other kidney function tests, physical examination findings, and patient history are essential for accurate diagnosis.

It’s important to note that the BUN ratio can be influenced by factors unrelated to kidney function, such as diet, hydration status, and liver function. A single abnormal value should be interpreted cautiously and typically requires follow-up testing.

Limitations and when to seek medical advice

While the BUN ratio calculator provides useful information, it has limitations:

  • It cannot diagnose specific medical conditions
  • Results should be interpreted by a healthcare professional
  • Normal ranges may vary between laboratories
  • The ratio is less reliable in certain populations (elderly, malnourished)

When to seek medical advice: If you have abnormal kidney function test results, symptoms like fatigue, swelling, changes in urination, or if you have risk factors for kidney disease (diabetes, hypertension, family history), consult a healthcare provider for proper evaluation and management.

Frequently asked questions

What is a normal BUN to creatinine ratio?

Typically 10–20; values >20 often suggest pre-renal causes; values <10 may suggest intrinsic renal causes.

Can dehydration raise the BUN ratio?

Yes — dehydration and reduced renal perfusion commonly increase the ratio as the kidneys try to conserve water, leading to increased urea reabsorption.

Are there medications that affect BUN or creatinine?

Yes — several medications can affect these values. Corticosteroids and tetracyclines may increase BUN, while cimetidine and trimethoprim can increase creatinine. Always review your medications with your healthcare provider.

How does diet affect the BUN ratio?

High protein intake can increase BUN levels, potentially raising the ratio. Conversely, low protein intake or malnutrition can lower BUN and decrease the ratio.

What’s the difference between BUN and creatinine?

BUN measures the amount of urea nitrogen in the blood, which is a waste product of protein metabolism. Creatinine is a waste product from muscle metabolism. Both are filtered by the kidneys, but they’re handled differently, which makes their ratio clinically useful.

References

  • National Kidney Foundation. (2023). Understanding Your Lab Values.
  • KDIGO Clinical Practice Guideline for Acute Kidney Injury. (2012).
  • Lyman, J. L. (1988). Blood urea nitrogen and creatinine. Emergency Medicine Clinics of North America.
  • K/DOQI clinical practice guidelines for chronic kidney disease. (2002). American Journal of Kidney Diseases.

Last reviewed: October 2025 | Reviewed by: Medical Review Team

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