Calculate the BUN/creatinine ratio from lab values or estimate BUN or creatinine when one value and the ratio are known in mg/dL or equivalent units.
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BUN Creatinine Ratio Formula
BUN/Creatinine Ratio = BUN (mg/dL) / Creatinine (mg/dL)
- BUN = blood urea nitrogen, in mg/dL
- Creatinine = serum creatinine, in mg/dL
If your lab reports urea instead of BUN, convert first: BUN (mg/dL) = urea (mmol/L) × 2.801, or BUN (mg/dL) = urea (mg/dL) × 0.4665. For creatinine in µmol/L, divide by 88.42 to get mg/dL. The ratio itself is unitless because both values are expressed in mg/dL.
Rearranged forms used by the calculator:
BUN = Creatinine × Ratio Creatinine = BUN / Ratio
Reference Ranges and Interpretation
Adult reference ranges vary slightly by lab. The values below are the commonly cited targets.
| Ratio | Category | Common considerations |
|---|---|---|
| < 10:1 | Low | Low protein intake, liver disease, SIADH, rhabdomyolysis, pregnancy |
| 10:1 to 20:1 | Typical | Within usual adult range |
| > 20:1 | High | Dehydration, prerenal azotemia, GI bleed, high-protein intake, catabolic state |
Typical lab reference values for the inputs:
| Marker | Adult male | Adult female |
|---|---|---|
| BUN | 8 to 24 mg/dL | 6 to 21 mg/dL |
| Creatinine | 0.74 to 1.35 mg/dL | 0.59 to 1.04 mg/dL |
| Urea (SI units) | 2.9 to 8.6 mmol/L | 2.1 to 7.5 mmol/L |
Worked Example
A patient has BUN of 28 mg/dL and creatinine of 1.4 mg/dL.
Ratio = 28 / 1.4 = 20:1. This sits at the top end of the typical range. If the same patient returns with BUN of 42 mg/dL and creatinine of 1.4 mg/dL, the ratio jumps to 30:1, which points toward a prerenal cause such as volume depletion rather than intrinsic kidney damage.
FAQ
Is BUN the same as urea? No. BUN measures only the nitrogen portion of urea. Urea is roughly 2.14 times the BUN value when both are in the same mass units.
Why must both values be in mg/dL? The ratio cancels units only when numerator and denominator share them. Mixing mg/dL with µmol/L gives a meaningless number.
Does the ratio replace eGFR? No. The ratio helps localize the cause of an abnormal BUN or creatinine. eGFR estimates kidney function. Use them together.
