BUN/Creatinine Ratio Calculator
What the BUN/Creatinine ratio is
The BUN/creatinine (BUN/Cr) ratio compares two common blood test results:
- BUN (blood urea nitrogen): a by‑product of protein metabolism produced in the liver and excreted by the kidneys.
- Creatinine: a by‑product of muscle metabolism that is also excreted by the kidneys.
Because both substances are filtered (and, for urea, partially reabsorbed) by the kidneys, the ratio can sometimes help clinicians interpret whether an elevated BUN and/or creatinine pattern is more consistent with reduced blood flow to the kidneys (prerenal causes), intrinsic kidney problems, or non‑kidney factors. The ratio is not diagnostic by itself and should be interpreted alongside symptoms, exam findings, urine tests, medications, and other labs (often including eGFR).
Formula
If your lab reports BUN and creatinine in the usual U.S. units of mg/dL, the calculation is simply:
Example: BUN 28 mg/dL and creatinine 1.1 mg/dL → ratio = 28 ÷ 1.1 ≈ 25.5.
Unit note: This calculator assumes both values are entered in mg/dL. If your report uses different units (e.g., mmol/L or µmol/L), convert first or use a unit‑aware tool; mixing units will give a meaningless ratio.
Typical ranges and what higher or lower ratios may mean
Many clinical references cite an approximate adult ratio range of 10:1 to 20:1, but reference ranges vary by lab and by clinical context. Use your lab’s provided reference interval and clinician guidance.
| Ratio (BUN/Cr) | Common interpretation (not diagnostic) | Examples of contributing factors |
|---|---|---|
| ~10–20 | Often considered “typical” in many adults | Results still depend on the absolute BUN/Cr values, hydration status, and lab ranges |
| >20 | May be consistent with prerenal patterns (reduced kidney perfusion) or increased urea production | Dehydration, vomiting/diarrhea, diuretics, heart failure, GI bleeding, high‑protein intake, corticosteroids |
| <10 | May occur when creatinine is relatively higher or urea production is lower | Low protein intake, severe liver disease (reduced urea production), rhabdomyolysis/increased creatinine, some intrinsic kidney disorders |
How to interpret your result responsibly
- Look at the full picture: a ratio can be “normal” while both numbers are abnormal, or “high” while creatinine is normal but BUN is elevated from diet/medications.
- Consider recent hydration and intake: dehydration can concentrate BUN more than creatinine, increasing the ratio.
- Consider muscle mass: low muscle mass can lower creatinine (raising the ratio), while very muscular individuals may have higher baseline creatinine (lowering the ratio).
- Compare to your prior results: trends are often more informative than a single point.
Worked example
Suppose your lab report shows:
- BUN: 28 mg/dL
- Creatinine: 1.1 mg/dL
Calculation: 28 ÷ 1.1 = 25.5.
Interpretation: A ratio around 25.5 is above the common 10–20 range. This pattern may be seen with dehydration or reduced blood flow to the kidneys, but it can also be influenced by non‑kidney factors (e.g., high protein intake, GI bleeding, certain medications). A clinician would typically correlate with symptoms (thirst, dizziness, reduced urine output), vital signs, urine studies, and additional labs before drawing conclusions.
Limitations and assumptions (important)
- Not medical advice / not a diagnosis: This calculator provides an estimate for educational use only and cannot diagnose kidney disease, dehydration, or any other condition.
- Units assumption: Inputs must both be in mg/dL.
- Lab variability: Reference ranges and methods differ by laboratory; use your lab’s interval.
- Population context: Interpretation differs for children, pregnant patients, older adults, and people with very low or very high muscle mass.
- Confounders: Diet (high protein), GI bleeding, steroids, catabolic states, liver disease, muscle injury, and many medications can change BUN and/or creatinine independent of filtration.
- Ratio alone is incomplete: Clinicians usually interpret BUN/Cr alongside creatinine trend, eGFR, urine output, urinalysis, and clinical status.
If you have concerning symptoms (confusion, severe weakness, chest pain, fainting, very low urine output, persistent vomiting/diarrhea), seek urgent medical care.
FAQ
What is a normal BUN/creatinine ratio?
Many sources cite roughly 10–20 for adults, but your lab’s reference range and your clinical context matter most.
What can cause a high BUN/Cr ratio?
A higher ratio can occur with dehydration or other prerenal states, and also with increased urea production (e.g., high‑protein diet, GI bleeding, corticosteroids). It is not specific.
What can cause a low BUN/Cr ratio?
A lower ratio can be seen with lower urea production (e.g., severe liver disease, low protein intake) or relatively higher creatinine (e.g., muscle injury). Interpretation depends on the absolute values.
Does dehydration affect the ratio?
Often yes. Dehydration may raise BUN more than creatinine, increasing the ratio, but this is not always the case.
Do the units matter?
Yes. Enter both values in mg/dL. If your report uses different units, convert them before calculating the ratio.