Divides the blood urea nitrogen by the creatinine, in US or SI units, and shows what a low, usual or high ratio can point to.
Result
Enter the BUN (or urea) and the creatinine.
Sources
4 studies
4
studies
172
citations
436
PubMed papers
Evidence
Known limitations
Among 20,126 hospital patients, 3,641 developed acute kidney injury; 53% of them had a ratio above 20. Hospital mortality was higher with a high ratio (29.9% vs 18.4%), the opposite of what a benign prerenal state would predict. The authors conclude that these findings do not support the ratio as a marker of prerenal azotemia.
Clin Kidney J: Journal Metrics Score 3.8, Q1 · cited 172 times
PubMed · Free full text · DOI
In 1,103 adult emergency department patients with acute kidney injury and creatinine above 1.5 mg/dL (prerenal defined as creatinine recovery within 7 days; obstruction, dialysis and transplant excluded), the mean ratio did not differ between prerenal and intrinsic injury, with an area under the ROC curve of 0.5.
BMC Nephrol: Journal Metrics Score 2.7, Q1 · cited 70 times
PubMed · Free full text · DOI
A systematic review of features that separate upper from lower gastrointestinal bleeding and predict severe upper bleeding: a ratio above 30 had a summary likelihood ratio of 7.5 (95% CI 2.8 to 12.0) for an upper source, along with melena and a bloody nasogastric aspirate.
In 325 patients with gastrointestinal bleeding without hematemesis, a ratio of 30 or more (odds ratio 10.0, 95% CI 4.0 to 25.6), black stool and age under 50 independently predicted an upper source.
Am J Emerg Med: Journal Metrics Score 2.1, Q1 · cited 54 times
The key study has been cited 172 times, and 436 PubMed articles mention the BUN/creatinine ratio in their title or abstract. Citation counts from OpenAlex and article counts from PubMed, retrieved November 9, 2026. Journal Metrics Scores and quartiles are from this site’s current release; follow the journal name for details.
After the result
Above 20 · 10 to 20 · Below 10
Look for volume loss, bleeding or a high protein load. Judge the urea and creatinine themselves. Low urea or high creatinine for the other.
Next steps
Avoid common errors
5 common errors to avoid
and 3 more
When the ratio misleads
1Not a test for prerenal injury
In 20,126 hospital patients (Uchino 2012), about half of those with acute kidney injury had a ratio above 20, and they did worse, not better. In 1,103 emergency patients (Manoeuvrier 2017), the ratio did not separate prerenal from intrinsic injury at all (area under the curve 0.5).
2Urea is not BUN
Urea in mg/dL is about 2.14 times the BUN. Entering urea in mg/dL as BUN more than doubles the ratio.
3Low creatinine
With a creatinine below about 0.5 mg/dL, as in children or frail adults, a small change in creatinine changes the ratio a lot.
4Diet and drugs
Protein loads, tube feeding, corticosteroids and tetracyclines raise urea without a change in filtration. Trimethoprim and cimetidine raise creatinine without a change in filtration, lowering the ratio.
5Severity of a bleed
A ratio above 30 favors an upper source in gastrointestinal bleeding, but it is not a measure of severity; use a validated score such as the Glasgow-Blatchford score, which includes the urea itself.
Formula
BUN (mg/dL) ÷ creatinine (mg/dL); usual 10 to 20.
BUN ÷ creatinine (mg/dL)
BUN (mg/dL) ÷ creatinine (mg/dL); usual 10 to 20.
Formula and ranges
| Step | Calculation |
|---|---|
| Ratio | BUN (mg/dL) ÷ creatinine (mg/dL) |
| Urea to BUN | urea (mmol/L) × 2.801 = BUN (mg/dL) |
| Creatinine | creatinine (µmol/L) ÷ 88.42 = mg/dL |
| Result | Ratio | Meaning |
|---|---|---|
| Low ratio | below 10 | Below 10: urea is low for the creatinine, as with low protein intake, liver disease or overhydration, or creatinine is high for the urea, as with muscle breakdown. |
| Within the usual range | 10 to 20 | Between 10 and 20: the usual range. It does not exclude kidney injury; judge the urea and creatinine themselves and their trend. |
| High ratio | above 20 | Above 20: urea is high for the creatinine, as with volume depletion, heart failure, an upper gastrointestinal bleed, high protein intake, corticosteroids, a catabolic state, urinary obstruction or low muscle mass. It does not reliably separate prerenal from intrinsic kidney injury. |
Why urea rises more
Upper GI bleeding
SI ratio
Units
Test cases
3 published cases, each also a test case
3 published cases; the calculator reproduces every ratio.
Worked examples
A 57-year-old man with peptic ulcer disease and previous bypass surgery presented with hematemesis and a heart rate of 105. His BUN was 32 mg/dL with a stable creatinine of 0.8 mg/dL, which the report gives as a ratio of 40.
“an elevated blood urea nitrogen (BUN) of 32 mg/dL with a stable creatinine of 0.8 mg/dL yielding a blood urea nitrogen-to-creatinine ratio of 40”
Blood digested in the gut is absorbed as protein, so urea rises while creatinine does not.
Entered as: BUN or urea: 32 mg/dL (BUN); Creatinine: 0.8 mg/dL.
Source: Tanyous A et al., Cureus 2026 (PubMed · Free full text · DOI)
A 36-year-old man with mast cell activation syndrome had intractable nausea and vomiting. BUN was 21 mg/dL and creatinine 0.85 mg/dL, both within the laboratory range; the report gives a ratio of 24.7, consistent with dehydration.
“a mildly elevated blood urea nitrogen (BUN)-to-creatinine ratio (24.7), consistent with dehydration secondary to the episodes of vomiting”
A high ratio with normal values of both: a high ratio can hint at volume depletion even when both values are within range.
Entered as: BUN or urea: 21 mg/dL (BUN); Creatinine: 0.85 mg/dL.
Source: Salib M et al., Cureus 2026 (PubMed · Free full text · DOI)
A 15-year-old girl was admitted after an amitriptyline overdose. Her BUN was 4 mg/dL and creatinine 0.74 mg/dL; the laboratory table gives a BUN/creatinine ratio of 5.41, below its range of 10 to 24.
“a low blood urea nitrogen (BUN) level, a low BUN/creatinine ratio, and mild hyperglycemia”
The calculator shows one decimal (5.41 before rounding). A low BUN rather than a high creatinine made the ratio low.
Entered as: BUN or urea: 4 mg/dL (BUN); Creatinine: 0.74 mg/dL.
Source: Navarro AW et al., Cureus 2026 (PubMed · Free full text · DOI)
FAQ
6 questions and answers
and 4 more answers
Questions
Divide the blood urea nitrogen by the creatinine, both in mg/dL. A BUN of 32 mg/dL and a creatinine of 0.8 mg/dL give 32 ÷ 0.8 = 40. With SI units, multiply urea in mmol/L by 2.801 to get BUN in mg/dL, and divide creatinine in µmol/L by 88.42 to get mg/dL; the calculator does both.
About 10 to 20 in most references, though laboratory ranges differ (one in the examples below uses 10 to 24, another 12 to 20). The ratio is only a clue: Uchino and colleagues found that about half of patients with acute kidney injury had a ratio above 20, and they had higher, not lower, mortality; Manoeuvrier and colleagues found it did not separate prerenal from intrinsic injury (area under the curve 0.5).
Urea is high for the creatinine. Common causes are volume depletion (vomiting, diarrhea, diuretics), an upper gastrointestinal bleed, a high-protein diet or feeding, corticosteroids, and catabolic states. In a patient with gastrointestinal bleeding, a ratio above 30 favors an upper source.
Urea is low for the creatinine: low protein intake, liver disease, overhydration or pregnancy (which lowers both) can lower urea, and muscle breakdown (rhabdomyolysis) can raise creatinine. A low ratio on its own is rarely a diagnosis.
It helps a little. In the Srygley 2012 review, a ratio above 30 had a summary likelihood ratio of 7.5 for an upper source in patients with gastrointestinal bleeding; Witting 2006 found a ratio of 30 or more independently predicted an upper source in patients without hematemesis. It does not grade how severe the bleed is.
Yes, in mmol/L: the calculator multiplies it by 2.801 to give BUN in mg/dL. Urea in mg/dL is different from BUN: divide it by 2.14 to get BUN. Countries using SI units sometimes quote the urea (mmol/L) to creatinine (µmol/L) ratio instead; a BUN/creatinine ratio of 20 is about 0.081 on that scale, or about 81 with creatinine in mmol/L, the form often quoted in the UK and Europe.
Further reading
7 references with PubMed and DOI links
7
references
5
free full text
From Uchino S et al., Clin Kidney J 2012 to the newest case reports.
References
Contributed by Dr. Meng Zhao (MD · Former Neurosurgeon · Medical AI Researcher), from the original publications listed here. Last updated November 9, 2026. LinkedIn
The calculator code is tested against every worked example on this page before each release.
This calculator supports, and does not replace, clinical judgment. It is meant for healthcare professionals and is not medical advice for patients. Report an error through the contact page.
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