The BUN/creatinine ratio compares two waste products that appear side by side on a standard metabolic panel: blood urea nitrogen (BUN) and creatinine. Both are cleared by the kidneys, but they come from different places and respond to different things, so looking at them together tells your doctor more than either number alone. A high ratio often points to something outside the kidneys, such as dehydration, a high-protein diet or bleeding in the upper digestive tract. A low ratio can reflect liver disease, a low protein intake or a high muscle mass. The ratio is a clue, not a diagnosis.
In this article you’ll learn how the ratio is calculated, what counts as a typical value, the common reasons for a high or low ratio, how it relates to kidney function, and when a result needs medical attention.
What the BUN/creatinine ratio measures
BUN measures urea nitrogen, a waste product that forms when your body breaks down protein; the kidneys remove it in urine, according to MedlinePlus. Creatinine is a waste product released as muscles are used, and the amount you make depends mostly on your muscle mass, age and activity level, as MedlinePlus explains. Both are filtered by the kidneys, but BUN is also strongly affected by how much protein you eat, how well hydrated you are and whether there is blood in your gut, while creatinine stays steadier.
That difference is what makes the ratio useful. If both values rise together, the kidneys themselves may be filtering less. If BUN rises much more than creatinine, the cause often lies elsewhere. Both tests are part of a routine metabolic panel, and MedlinePlus lists the BUN to creatinine ratio as another name for the BUN test. To review each value in detail, read our BUN blood test guide, check our 肌酐血液检测指南 and open our comprehensive metabolic panel guide.
How the ratio is calculated
The calculation is simple: divide the BUN by the creatinine. With a BUN of 18 mg/dL and a creatinine of 0.9 mg/dL, the ratio is 18 ÷ 0.9 = 20. Many US reports calculate it automatically and print it on the line below the two results.
Units change the number completely. In the United States, both values are usually reported in milligrams per deciliter (mg/dL). Many other countries measure urea in millimoles per liter and creatinine in micromoles per liter, which produces much larger numbers for the same person. A 2025 review of hospital studies noted that a BUN-to-creatinine ratio of 20 in US units corresponds to a urea-to-creatinine ratio of about 80 when both are converted to millimoles per liter (Paulus et al., 2025). Always check which units your report uses before comparing it with ranges you read online.
Normal range for the BUN/creatinine ratio
There is no single standard range for the BUN/creatinine ratio. Some laboratories print a reference range next to it, and that is the range to use. In research using US units, a value around 20 is commonly used as the dividing line between a typical and a raised ratio. For context, 梅奥诊所 describes a BUN of about 7 to 20 mg/dL as generally healthy, and 克利夫兰诊所 gives adult ranges of 6 to 21 mg/dL for women and 8 to 24 mg/dL for men; creatinine ranges depend heavily on muscle mass and sex.
Because the ratio divides one number by another, it can look abnormal when both values are normal. A slightly high BUN combined with a low creatinine, common in small or older adults with less muscle, can produce a ratio above 20 without any illness. The pattern of the two numbers matters more than the ratio alone, as the table shows.
| 报告中的指标模式 | Ratio | 常见原因 |
|---|---|---|
| BUN high, creatinine normal | 高的 | Dehydration, high protein intake, bleeding in the stomach or upper intestine, steroids and some other medicines, burns, heart failure |
| BUN and creatinine both high | Often normal or mildly high | Reduced kidney filtration, from either a kidney problem or poor blood flow to the kidneys; needs evaluation |
| BUN normal, creatinine low | 高的 | Low muscle mass, older age, small body size; often not a sign of disease |
| BUN low, creatinine normal | 低的 | Liver disease, low protein intake, malnutrition, drinking a lot of fluid |
| BUN normal, creatinine high | 低的 | High muscle mass, a recent large meat meal, intense exercise or kidney disease |
Getting a reliable result
Small, everyday factors can move the ratio from one test to the next, so a single value is best read as a snapshot. Before a repeat test, it usually helps to drink your normal amount of fluid, avoid an unusually large meat or protein meal the day before and skip very intense exercise, since MedlinePlus notes that eating meat, hard exercise and some supplements can briefly raise creatinine. Tell the person ordering the test about any medicines and supplements you take, including steroids, because these can shift BUN or creatinine. If you have recently been unwell with vomiting, diarrhea or fever, mention it, as fluid loss raises BUN. Comparing results from the same laboratory over time is also more reliable than comparing reports from different labs, which may use different units and reference ranges. A BUN/creatinine ratio that is high once and normal on a repeat test under these conditions usually reflects a temporary state rather than a lasting problem.
What a high BUN/creatinine ratio means
A high ratio usually means BUN has risen more than creatinine. MedlinePlus, Mayo Clinic and Cleveland Clinic together list the main reasons BUN goes up: dehydration, a high-protein diet, bleeding in the stomach or intestines, heart failure, a recent heart attack, shock, severe burns, a blocked urinary tract, aging, stress and certain medicines.
脱水与肾脏血流减少
When you are short of fluid, the kidneys hold on to water and reabsorb more urea along with it, so BUN climbs faster than creatinine. Vomiting, diarrhea, heavy sweating, fever, not drinking enough and water pills can all do this. Conditions that reduce blood flow to the kidneys, such as heart failure or shock, have a similar effect. If dehydration is the cause, the ratio usually returns to normal once you are rehydrated. Learn the warning signs by reading our dehydration guide, and for the heart side of the picture, explore our heart failure diagnosis guide.
Bleeding in the upper digestive tract
Blood that leaks into the stomach or upper intestine is digested like a high-protein meal, so the gut absorbs the breakdown products and the liver turns them into urea. That is why gastrointestinal bleeding appears on every major list of causes of a high BUN. A high ratio alongside black, tarry stools, vomiting blood or a falling hemoglobin is a reason for urgent care. A common source is a stomach or duodenal ulcer; to learn how ulcers bleed, read our peptic ulcer guide. To see how blood loss shows up on a blood count, consult our hemoglobin levels guide.
Protein, medicines and other causes
Eating a lot of protein raises BUN, and Mayo Clinic notes that the test can show whether protein intake is appropriate. Corticosteroids such as prednisone can raise BUN, and Cleveland Clinic also lists carbamazepine, methotrexate and tetracycline. Severe burns, major physical stress and a recent heart attack can also push the ratio up. Do not stop a prescribed medicine because of your ratio; discuss it with your doctor.
A high ratio when everything else is normal
This is one of the most common questions people ask after seeing their results. If BUN and creatinine are each within range and only the ratio is flagged, the usual explanations are mild dehydration on the day of the test, a protein-rich diet or a naturally low creatinine from lower muscle mass. Most doctors simply repeat the test after normal fluid intake rather than ordering further investigations, unless there are symptoms.
What a low BUN/creatinine ratio means
A low ratio means BUN is low compared with creatinine. The causes of a low BUN listed by MedlinePlus, Mayo Clinic and Cleveland Clinic include liver disease, a low-protein diet, malnutrition and overhydration. The liver makes urea, so when it is badly damaged, BUN can be low even when the kidneys work normally, according to the MedlinePlus medical encyclopedia. To understand the liver side, read our cirrhosis guide.
A low ratio can also come from a higher creatinine. MedlinePlus notes that people with more muscle make more creatinine, and that eating meat, intense exercise and certain supplements can raise it temporarily. A muscular person with a normal BUN may therefore have a ratio below 10 without any illness.
The BUN/creatinine ratio and kidney function
The ratio is not a measure of kidney function on its own. Mayo Clinic explains that BUN and creatinine usually both rise when kidney function declines, and that the care team interprets them together with eGFR, electrolytes and urine albumin. Cleveland Clinic adds that healthcare providers do not use BUN to define kidney failure. The estimated glomerular filtration rate, calculated from creatinine, age and sex, is the standard measure; to understand your filtration number, review our eGFR血液检测指南.
When creatinine is unreliable, for example in people with very low or very high muscle mass, doctors may add a cystatin C test, which does not depend on muscle; see our 胱抑素C血液检测指南. If both BUN and creatinine stay high on repeat tests, the next steps focus on kidney health; for the long-term picture, study our 慢性肾病指南.
何时应该去看医生
Seek emergency care if a high ratio, or any blood test result, comes with black or tarry stools, vomiting blood or material that looks like coffee grounds, fainting, chest pain, confusion or very little urine. Contact your doctor soon if any of the following applies:
- Signs of dehydration that do not improve with fluids, such as dizziness, a dry mouth and dark urine.
- Swelling of the legs or shortness of breath alongside abnormal kidney results.
- BUN and creatinine that are both high, or an eGFR that has dropped.
- A high ratio that stays high on a repeat test after you have been drinking normally.
- A low ratio together with yellowing of the skin, swelling of the belly or unexplained weight loss.
A single mildly high or low ratio with normal BUN, creatinine and eGFR, and no symptoms, is usually rechecked rather than investigated.
最新科学进展
Recent research shows where the BUN/creatinine ratio helps and where it falls short.
It can hint at where bleeding comes from
A 2026 analysis pooling 17 studies found that people bleeding from the upper digestive tract, the esophagus, stomach or first part of the intestine, had clearly higher ratios than people bleeding lower down. A ratio of 22 or more correctly pointed to an upper source about two thirds of the time, and a ratio above 30 was more specific but missed many cases. What this means for you: in someone with signs of bleeding, a high ratio helps doctors decide where to look first, but it never replaces an endoscopy.
Kidney problems blur the signal
A 2025 study of 849 people with confirmed digestive bleeding found that the ratio distinguished upper from lower bleeding reasonably well when kidney function was normal, but much less well in people with chronic kidney disease or sudden kidney injury. What this means for you: if your kidneys are not working normally, your doctor will give your ratio less weight.
Not a reliable test for dehydration on its own
A 2026 study of 921 people fasting before surgery compared the ratio with four urine measures of hydration, such as urine color and concentration, and found no link between them. The authors concluded that the ratio may not reliably reflect hydration status. What this means for you: a high ratio is a reason to think about fluids, but doctors also look at your symptoms, blood pressure, sodium and urine tests.
In intensive care, a rising ratio can signal muscle breakdown
A 2025 review of 47 studies in critically ill adults found that a rising ratio during a long intensive care stay often tracks muscle breakdown, and that a ratio of 20 or more on admission was linked to a higher risk of dying in hospital. Dehydration, bleeding, kidney and liver problems can all distort it. What this means for you: if a relative is in intensive care, the ratio is one of many numbers the team uses to adjust nutrition and fluids, not a verdict on its own.
词汇表
| 术语 | 定义 |
|---|---|
| 血尿素氮（BUN） | The amount of nitrogen from urea in the blood; urea forms when the body breaks down protein. |
| 肌酐 | A waste product made by muscles at a fairly steady rate and removed by the kidneys. |
| BUN/肌酐比值 | BUN divided by creatinine, used to judge whether a high BUN is likely to come from inside or outside the kidneys. |
| eGFR | Estimated glomerular filtration rate, the standard estimate of how well the kidneys filter blood. |
| 胱抑素C | A protein used as an alternative kidney filtration marker that does not depend on muscle mass. |
| Upper gastrointestinal bleeding | Bleeding from the esophagus, stomach or first part of the small intestine. |
| 脱水 | A shortage of body water, from losing more fluid than you take in. |
| Muscle catabolism | The breakdown of muscle protein, which happens during serious illness and raises urea. |
| mg/dL 与 mmol/L | Two systems of laboratory units; the same blood sample gives very different numbers depending on which one is used. |
常见问题
What is a dangerously high BUN/creatinine ratio?
There is no single dangerous number, because the meaning depends on the BUN and creatinine values themselves and on your symptoms. A ratio far above 20 together with signs of bleeding, severe dehydration, confusion or very little urine needs urgent care. A ratio slightly above 20 with normal BUN and creatinine in someone who feels well is usually not dangerous and is often rechecked.
What should I do if my BUN/creatinine ratio is high?
Look at the BUN and creatinine lines first. If both are normal and you feel well, drink normally, avoid an unusually high-protein meal before the next test and ask your doctor whether to repeat it. If BUN or creatinine is high, or you have symptoms such as black stools, vomiting, dizziness or swelling, contact your doctor promptly so the cause can be found.
Can dehydration cause a high BUN/creatinine ratio?
Yes. Dehydration is one of the most common reasons. When you are short of fluid, the kidneys reabsorb more urea, so BUN rises more than creatinine. The ratio usually improves once you are rehydrated. Research suggests the ratio alone is not a reliable measure of hydration, so doctors also consider your symptoms and other tests.
What does a low BUN/creatinine ratio mean?
A low ratio means BUN is low compared with creatinine. Common reasons include a low-protein diet, drinking a lot of fluids, liver disease, malnutrition and a high muscle mass, which raises creatinine. A low ratio on its own, with normal kidney results and no symptoms, is rarely a concern.
Can diet affect my BUN/creatinine ratio?
Yes. A high-protein diet or protein supplements can raise BUN and therefore the ratio, while a very low-protein diet can lower it. A large meat meal shortly before the test can raise creatinine temporarily. If your result is borderline, your doctor may ask you to avoid unusual meals before a repeat test.
Is the BUN/creatinine ratio the same as a kidney function test?
Not exactly. BUN and creatinine are both part of kidney testing, but the ratio mainly helps explain why BUN is high or low. Kidney function is judged mainly by eGFR, calculated from creatinine, together with urine albumin. A normal ratio does not rule out kidney disease, and an abnormal ratio does not prove it.
参考来源
- MedlinePlus, National Library of Medicine — BUN (Blood Urea Nitrogen) — MedlinePlus Lab Tests, updated 2024 — medlineplus.gov
- MedlinePlus Medical Encyclopedia, National Library of Medicine — BUN blood test — reviewed 2025 — medlineplus.gov
- MedlinePlus, National Library of Medicine — Creatinine Test — MedlinePlus Lab Tests, updated 2023 — medlineplus.gov
- Cleveland Clinic — Blood Urea Nitrogen (BUN) Test — Health Library, reviewed 2026 — my.clevelandclinic.org
- Mayo Clinic Staff — Blood urea nitrogen (BUN) test — Mayo Clinic, 2026 — mayoclinic.org
- Paulus MC, Melchers M, van Es A, Kouw IWK, et al. — The urea-to-creatinine ratio as an emerging biomarker in critical care: a scoping review and meta-analysis — Critical Care, 2025 — doi.org/10.1186/s13054-025-05396-6
- Simadibrata DM, Koo TH, Murlidhar M, Lamichhane R, et al. — Blood Urea Nitrogen-to-Creatinine Ratio to Differentiate Upper From Lower Gastrointestinal Bleeding: A Systematic Review and Meta-Analysis — Journal of Gastroenterology and Hepatology, 2026 — doi.org/10.1111/jgh.70224
- Russ P, Koppenhöfer JM, Bedenbender S, Tarawneh TS, et al. — Diagnostic value of the urea-to-creatinine ratio for gastrointestinal bleeding source: influence of renal function — BMC Nephrology, 2025 — doi.org/10.1186/s12882-025-04382-y
- Li Y, Hahn RG — Comparison between the blood urea nitrogen (BUN)-to-creatinine ratio and four urinary biomarkers of dehydration; a cross-sectional study in fasting patients awaiting surgery — Scandinavian Journal of Clinical and Laboratory Investigation, 2026 — doi.org/10.1080/00365513.2026.2713141
延伸阅读
- Check another blood measure of hydration by reading our serum osmolality guide.
- Understand the electrolyte that shifts with fluid balance by reviewing our 血钠检测指南.
- Follow the protein your liver makes by opening our 白蛋白血液检测指南.
- Decode the urine test that catches early kidney damage by exploring our 白蛋白肌酐比值指南.
- Learn what another kidney-cleared waste product reveals by visiting our 尿酸血液检测指南.
用BloodSense轻松看懂您的化验结果
Your BUN/creatinine ratio makes sense only next to the BUN, creatinine and eGFR values it comes from, and to your hemoglobin and electrolytes. BloodSense helps you understand your metabolic panel and kidney results in plain language, with the questions worth asking your doctor. It does not make a diagnosis and does not replace your doctor.
