Two waste products, one ratio
Blood urea nitrogen (BUN) and creatinine are both waste products that the kidneys filter. Creatinine comes from muscle at a fairly steady rate and is not reabsorbed. Urea comes from protein breakdown and is partly reabsorbed in the kidney, more so when blood flow to the kidneys is low. Comparing the two helps sort out why kidney tests are abnormal: the BUN to creatinine ratio.
The formulas
- Ratio (US units) = BUN (mg/dL) ÷ creatinine (mg/dL)
- SI ratio = urea (mmol/L) ÷ creatinine (mmol/L)
- Urea (mmol/L) = BUN (mg/dL) ÷ 2.8
- Creatinine (µmol/L) = creatinine (mg/dL) × 88.4
Because the units differ, the SI ratio is about four times the US ratio: a US ratio of 15 corresponds to an SI ratio of about 61.
Worked example
A metabolic panel shows a BUN of 15 mg/dL and a creatinine of 1.0 mg/dL.
- Ratio = 15 ÷ 1.0 = 15, in the usual range.
- In SI units, urea = 15 ÷ 2.8 = 5.36 mmol/L and creatinine = 88 µmol/L.
- SI ratio = 5.36 ÷ 0.088 = 61.
Interpretation
| BUN/creatinine | Possible causes |
|---|---|
| Below 10 | Liver disease, low protein intake, malnutrition, pregnancy, intrinsic kidney damage, rhabdomyolysis |
| 10 to 20 | Normal, or kidney disease affecting both values equally |
| Above 20 | Reduced kidney blood flow (dehydration, heart failure, shock), upper GI bleeding, high protein intake, corticosteroids |
A ratio above 20 with a raised creatinine is the classic pattern of prerenal kidney injury, where the kidneys are healthy but underperfused. In upper gastrointestinal bleeding, digested blood is a large protein load, so BUN rises out of proportion to creatinine.
Caveats
The ratio only helps when at least one of the values is abnormal: with a normal BUN and creatinine, a ratio of 22 means little. Creatinine depends on muscle mass, so older adults, people with low muscle mass and amputees can have a high ratio with low creatinine. The ratio does not replace the estimated glomerular filtration rate (eGFR), which is the main measure of kidney function.
This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.