BUN/Creatinine ratio
BUN/creatinine is the ratio of blood urea nitrogen (BUN) to creatinine. This simple marker helps reveal the nature of impaired kidney function: whether it is related to blood flow to the kidneys (pre-renal), to the kidney tissue itself (renal) or to urine outflow (a post-renal cause).
What BUN/creatinine shows
Both urea (through blood urea nitrogen, BUN) and creatinine are cleared by the kidneys, but they behave differently. When blood flow through the kidneys drops, urea is reabsorbed more than creatinine — and their ratio rises. So the BUN/creatinine ratio itself helps reveal not “how much” kidney function is impaired (there is eGFR for that) but where the cause lies.
Three types of cause
- Pre-renal, BUN/Crea > 20. The kidneys are healthy, but little blood reaches them: dehydration, blood loss, heart failure, a high-protein diet.
- Renal (kidney), BUN/Crea < 10. The kidney tissue itself is damaged (for example, acute tubular necrosis, glomerulonephritis).
- Post-renal, BUN/Crea 10–20. An obstruction to urine outflow (a stone, an enlarged prostate) — but the normal range also falls into this window.
What's needed for the calculation
- Serum creatinine, µmol/L. From the blood chemistry panel.
- Serum urea, mmol/L. Also from the blood chemistry. The calculator will convert the values into the required units (BUN, mg/dL) on its own.
BUN/Creatinine calculator
Enter urea and creatinine — the calculator will compute the BUN/creatinine ratio and suggest the likely nature of impaired kidney function. The calculation runs on the server.
< 10 — renal cause · 10–20 — normal / post-renal · > 20 — pre-renal cause.
How to read the result
| BUN/Crea | Likely cause |
|---|---|
| < 10 | Renal (kidney) — damage to the kidney tissue |
| 10 – 20 | Normal or post-renal (an obstruction to outflow) |
| > 20 | Pre-renal (reduced blood flow to the kidneys) |
The cause is in the kidney tissue itself. Assessment of kidney function and a urinalysis are needed.
Often a normal variant; if there are symptoms — rule out an obstruction to urine outflow.
Reduced renal blood flow: dehydration, blood loss, heart failure.
Limitations and important caveats
- This is not a diagnosis. The ratio only indicates the direction of the search for a cause.
- It depends on many factors. A high-protein diet, gastrointestinal bleeding and steroid use raise urea; a low-protein diet and liver disease lower it.
- Assess it together with eGFR. On its own the ratio says nothing about the degree of decline in kidney function.
- The normal range and a post-renal cause overlap. The 10–20 window does not distinguish them — the clinical picture is needed.
- It does not replace an examination. The result is interpreted by a physician.
Frequently asked questions
How does BUN differ from urea?
BUN (blood urea nitrogen) is the mass of nitrogen contained in urea. The calculator converts urea (mmol/L) into BUN (mg/dL) itself to compute the ratio by the international standard.
In what units should the values be entered?
Urea in mmol/L and creatinine in µmol/L, as on the blood chemistry report. The calculator will convert to mg/dL itself.
My ratio is high — is that dangerous?
Most often a high value points to dehydration or another pre-renal cause rather than kidney disease. But it should be interpreted with a physician together with eGFR and a urinalysis.
What should I do with the result?
Show it to your physician along with your other tests. The ratio is a supportive pointer that helps the physician understand the nature of the impairment more quickly.
