Lab result explainedKidneyLast updated November 1, 2026

BUN/creatinine ratio: what a high or low ratio means

BUN/creatinine ratio (BUN/Cr ratio) · Comprehensive metabolic panel (CMP) · reported in ratio (BUN mg/dL divided by creatinine mg/dL)

The BUN/creatinine ratio is simply your blood urea nitrogen (BUN) divided by your creatinine, two waste products the kidneys clear, and it helps show whether a kidney number is off because of the kidneys themselves or because of something upstream, such as dehydration or bleeding. Most labs call 10:1 to 20:1 normal (some print 6 to 24). The two most common reasons it runs high are dehydration and a recent high-protein intake; the most common reasons it runs low are low muscle mass pushing creatinine down, or a liver that is making less urea.

The ratio is useful because BUN and creatinine behave differently. Urea is filtered by the kidney and then partly reabsorbed, and that reabsorption rises when the body is holding on to water. Creatinine is filtered and barely reabsorbed. So when blood flow to the kidneys falls, BUN climbs faster than creatinine and the ratio widens. When the kidneys themselves are damaged, both rise together and the ratio stays closer to normal.

A ratio on its own is rarely a diagnosis. Clinicians read it alongside the absolute BUN, creatinine and eGFR, plus your fluid status, diet, medicines and symptoms. This page explains what moves the ratio in each direction, how worried to be, and what usually happens next.

BUN/Cr ratio at a glance

  • What it is: blood urea nitrogen divided by creatinine, both in mg/dL.
  • Normal: roughly 10:1 to 20:1 on most US reports; many labs accept 6 to 24.
  • High (above 20): usually dehydration, high protein intake, bleeding in the gut, steroids or heart failure; sometimes low muscle mass.
  • Low (below 10): low protein intake, liver disease, pregnancy, overhydration, or a creatinine raised by muscle injury or certain drugs.
  • It matters less than eGFR: a normal ratio with a low eGFR is still a kidney problem.
  • Next step: repeat the test well hydrated, and look at BUN, creatinine, eGFR and urine together.

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What BUN/Cr ratio measures

BUN measures the nitrogen carried by urea, which the liver makes when it breaks down protein from food and from your own tissues. Creatinine is produced steadily by muscle as creatine is used for energy. Both leave the body almost entirely through the kidneys, which is why they are the standard screen for kidney function.

The lab does not run a separate test for the ratio. It reports BUN (typically 7 to 20 mg/dL) and creatinine (roughly 0.6 to 1.3 mg/dL) and divides one by the other. Because creatinine is a small number, modest shifts in it swing the ratio a lot: a creatinine of 0.6 with a BUN of 15 gives 25, while the same BUN with a creatinine of 1.0 gives 15. That is why a person with little muscle can have a high ratio without any kidney or fluid problem.

Outside the US, labs report urea in mmol/L and creatinine in micromol/L and use a urea:creatinine ratio with a different normal range (about 40 to 100). Make sure you are comparing like with like before you interpret a number from another country.

Normal range for BUN/Cr ratio

Reference ranges vary between labs and by age, sex and pregnancy. Use the range printed on your own report.

GroupTypical range (ratio (BUN mg/dL divided by creatinine mg/dL))Notes
Adults, typical10:1 to 20:1Many labs print 6 to 24 as the accepted range.
Older adultsOften 15 to 24Lower muscle mass lowers creatinine and raises the ratio without disease.
ChildrenUsually 10 to 20 but creatinine is low, so small changes matterInterpret with a pediatric clinician.
PregnancyOften below 10Both BUN and creatinine fall as kidney filtration rises.

High BUN/creatinine ratio

A ratio above 20 usually means the kidneys are receiving less blood flow or more urea than usual, not that they are damaged. The classic pattern is a prerenal state: dehydration, blood loss or heart failure makes the kidney hold on to water and salt, and urea is reabsorbed along with it while creatinine is not.

The ratio also rises when urea production goes up. A big protein meal, a high-protein diet, corticosteroids, a burn or infection that breaks down muscle, or blood digested in the gut after an upper GI bleed all deliver extra nitrogen to the liver.

A ratio above 30 with an otherwise normal creatinine is a well-known clue to bleeding higher in the digestive tract, especially if you have black stools or feel faint. If the creatinine is also raised, the picture may be an acute kidney injury that started from low blood flow and needs prompt attention.

Common causes

  1. Dehydration

    Low fluid intake, vomiting, diarrhea, sweating or diuretics reduce blood volume. The kidney reabsorbs more water and urea follows it back into the blood, so BUN rises faster than creatinine.

  2. High protein intake

    Protein is broken down to amino acids and the surplus nitrogen is turned into urea. A high-protein diet or protein supplements can push BUN up by several mg/dL while creatinine stays flat.

  3. Low muscle mass

    Older age, frailty, amputation or long illness lower creatinine. Dividing a normal BUN by a small creatinine produces a high ratio with healthy kidneys.

  4. Upper gastrointestinal bleeding

    Blood in the stomach and small intestine is digested like a protein meal and absorbed as urea. A ratio above 30 raises the likelihood of a bleed above the colon several-fold.

  5. Heart failure

    A weak heart delivers less blood to the kidneys, which respond as if the body were dry. Urea reabsorption rises and the ratio widens even when total body water is high.

  6. Corticosteroids and tetracyclines

    Prednisone and similar drugs speed protein breakdown, and older tetracyclines have an anti-anabolic effect, both raising urea production.

  7. Urinary obstruction

    An enlarged prostate, stone or tumor that blocks urine flow slows the passage of filtrate, allowing more urea to be reabsorbed before it leaves.

  8. Catabolic illness

    Severe infection, burns, trauma or fever break down body protein. Urea output can double during the acute phase.

Symptoms that can go with it

  • Thirst, dry mouth, dark urine or dizziness on standing (dehydration)
  • Black, tarry stools, vomiting blood, or new breathlessness and pallor (bleeding)
  • Swollen ankles, breathlessness lying flat, weight gain (heart failure)
  • Weak urine stream or trouble emptying the bladder (obstruction)
  • Often none at all: a mildly high ratio is frequently an incidental finding

Low BUN/creatinine ratio

A ratio below 10 means BUN is low relative to creatinine. Most often the liver is making less urea because protein intake is low or liver function is impaired, or extra water has diluted the blood.

The other way to get a low ratio is a creatinine that is high for reasons unrelated to kidney filtration: a muscle injury that releases creatinine, a creatine supplement, or a drug that blocks creatinine from being secreted into the urine. In those cases the eGFR may read low while the kidneys are fine.

On its own a low ratio is rarely dangerous. It becomes important when the low BUN comes from advanced liver disease or severe malnutrition, or when a rising creatinine is pulling the ratio down.

Common causes

  1. Low protein intake

    Very low-protein diets, fasting, anorexia or poor appetite give the liver little nitrogen to convert to urea, so BUN falls.

  2. Liver disease

    Cirrhosis and acute liver failure reduce the urea cycle enzymes that make urea. BUN can be unusually low even as ammonia rises.

  3. Pregnancy

    Blood volume and kidney filtration both increase, lowering BUN and creatinine, with BUN falling proportionally more in the first trimesters.

  4. Overhydration or SIADH

    Excess water from intravenous fluids, compulsive drinking or inappropriate antidiuretic hormone dilutes urea more than creatinine.

  5. Rhabdomyolysis or muscle injury

    Crush injury, extreme exercise or statin myopathy release creatinine and creatine from damaged muscle, raising creatinine out of proportion to BUN.

  6. Creatine supplements and heavy muscle mass

    Creatine is converted to creatinine at a steady rate; a 5 g daily dose can raise creatinine by about 0.1 to 0.3 mg/dL and pull the ratio down.

  7. Drugs that block creatinine secretion

    Trimethoprim, cimetidine and fenofibrate stop the tubules from secreting creatinine, raising the blood level by 0.2 to 0.3 mg/dL without changing true filtration.

  8. Dialysis

    Urea is removed more efficiently than creatinine by dialysis, so post-dialysis ratios are often low.

Symptoms that can go with it

  • Usually none; a low ratio is most often found incidentally
  • Fatigue, muscle wasting or unintended weight loss (malnutrition)
  • Yellowed skin, abdominal swelling, easy bruising or confusion (liver disease)
  • Severe muscle pain and dark urine after extreme exertion or injury (rhabdomyolysis)

Read it alongside

  • Creatinine and eGFR — the absolute values show whether filtration itself is reduced; a high ratio with a normal eGFR is almost always prerenal or dietary.
  • BUN — a raised BUN with a raised ratio points to fluid or protein; a raised BUN with a normal ratio points to kidney disease.
  • Sodium and CO2 (bicarbonate) — high sodium supports dehydration, low bicarbonate suggests true kidney injury or acidosis.
  • Hemoglobin and hematocrit — a falling hemoglobin with a ratio above 30 strongly suggests gut bleeding.
  • Urinalysis and urine sodium — concentrated urine with low sodium confirms the kidney is conserving water as expected in a prerenal state.
  • Albumin and liver enzymes — low albumin, raised bilirubin or abnormal ALT help explain a low ratio from liver disease or malnutrition.

What usually happens next

  1. If the ratio is mildly high and you feel well, your clinician will usually repeat the CMP after a few days of normal fluid intake and before, not after, a protein-heavy meal.
  2. Expect questions about diuretics, steroids, NSAIDs, protein powders, creatine, recent vomiting or diarrhea, and any black stools.
  3. If creatinine is also up, the next tests are usually a urinalysis, urine protein-to-creatinine ratio and sometimes a kidney ultrasound to exclude obstruction.
  4. A ratio above 30 with anemia or dark stools leads to a stool blood test and often an endoscopy to look for a bleeding source.
  5. A low ratio with abnormal liver tests prompts a liver work-up; a low ratio with high creatinine prompts a muscle enzyme (CK) check and a review of supplements.
  6. Ask your clinician to look at the eGFR trend, not just the single ratio, and whether a cystatin C test would clarify your true kidney function.

Diet, lifestyle and natural levers with evidence

What can move BUN/Cr ratio once the cause is known. Nothing here replaces finding the cause.

Steady hydration before testing

Moderate evidence

Dehydration is the most common reason the ratio runs high, and fluid replacement reliably brings a prerenal BUN down within a day or two. In the CKD WIT trial, coaching 631 adults with kidney disease to drink about 1 extra litre a day did not slow decline in eGFR, so hydration corrects a dry sample rather than improving kidney function itself.

People with heart failure, advanced kidney disease or on fluid restriction should follow their clinician's limits.

Moderate, well-spread protein

Moderate evidence

Urea production tracks protein intake, and crossover studies show BUN rises within hours of a high-protein meal. Keeping intake near 0.8 to 1.2 g per kg of body weight, spread across the day, keeps BUN in range in most healthy adults. Very low-protein diets below 0.6 g/kg lower BUN but risk muscle loss and are only used in CKD under dietitian supervision.

Pause creatine and check its timing

Moderate evidence

Creatine is safe for the kidneys in healthy adults according to the International Society of Sports Nutrition position stand, but it raises serum creatinine and lowers the BUN/creatinine ratio. Stopping creatine for 7 to 14 days before a test removes this artefact; tell your clinician if you take it so the result is read correctly.

Limit alcohol and treat the liver cause

Moderate evidence

Alcohol-related liver disease reduces urea synthesis and is a leading cause of a persistently low ratio. Observational and intervention data show abstinence improves liver synthetic function over months in early disease. A low ratio from liver disease is not fixed with supplements; the lever is the underlying cause.

Stopping heavy drinking abruptly can cause withdrawal; do it with medical support.

Plant-forward eating for long-term kidney protection

Moderate evidence

Cohort studies following tens of thousands of adults show DASH and Mediterranean-style diets are linked to 14 to 30 percent lower risk of developing kidney disease. These patterns do not change the ratio directly but reduce the chance that an abnormal kidney number becomes a chronic one.

When to see a clinician promptly

  • Black or bloody stools, vomiting blood, or feeling faint with a ratio above 30: this can signal a gastrointestinal bleed and needs same-day care.
  • A ratio above 20 together with a creatinine that has risen by 0.3 mg/dL or more from your baseline, or very little urine output.
  • Signs of serious dehydration: confusion, rapid heartbeat, inability to keep fluids down, or no urine for 8 hours.
  • Breathlessness, leg swelling and weight gain over days, which can mean heart failure is reducing kidney blood flow.
  • A low ratio with yellow skin, abdominal swelling or confusion, which can point to liver failure.
  • Severe muscle pain with cola-coloured urine after exertion or a fall, which can mean rhabdomyolysis.

Frequently asked questions

What is a normal BUN/creatinine ratio?

Most US labs consider 10:1 to 20:1 normal, and many print 6 to 24 as the acceptable range. Healthy older adults often sit at 15 to 24 because they have less muscle and therefore a lower creatinine. A single value a few points outside the range, with a normal creatinine and eGFR, is usually not a problem.

Is a BUN/creatinine ratio of 25 bad?

Not by itself. A ratio of 25 with a normal creatinine most often reflects mild dehydration, a recent high-protein meal, low muscle mass or a steroid medicine. Your clinician will look at your BUN, creatinine and eGFR and usually repeat the test when you are well hydrated. It needs more attention if you have symptoms of bleeding, heart failure or a rising creatinine.

Can dehydration raise the BUN/creatinine ratio?

Yes, and it is the single most common reason. When blood volume falls the kidneys reabsorb more water and urea comes with it, so BUN rises while creatinine changes little. A ratio above 20 that returns to normal after rehydration confirms this.

What does a low BUN/creatinine ratio mean?

A ratio under 10 usually means low urea production from a low-protein diet or liver disease, or dilution from extra fluids or pregnancy. It can also come from a creatinine that is raised for muscle-related reasons, such as a creatine supplement, intense training or muscle injury. It is rarely dangerous on its own.

Does a high BUN/creatinine ratio mean kidney failure?

Usually not. Kidney failure typically raises BUN and creatinine together and keeps the ratio near normal. A high ratio points toward a cause outside the kidney, such as dehydration, bleeding or heart failure. The number that tells you about kidney failure is the eGFR.

Can a high-protein diet or protein shakes change the ratio?

Yes. Protein is converted to urea, and a high-protein diet or a large protein meal in the hours before the test can push BUN up by several mg/dL. Creatinine is unaffected, so the ratio climbs. Test fasting, or at least avoid a protein-heavy meal that morning, for a cleaner result.

Why is the BUN/creatinine ratio high in a GI bleed?

Blood that enters the stomach or small intestine is digested like a steak dinner. Its protein is absorbed and converted to urea, raising BUN, while creatinine stays normal. A ratio above 30 in someone with black stools or a falling hemoglobin makes an upper GI bleed considerably more likely.

References

  1. BUN (Blood Urea Nitrogen) Test. MedlinePlus, US National Library of MedicineView source
  2. Blood urea nitrogen (BUN) test. Mayo ClinicView source
  3. Creatinine Test. MedlinePlus, US National Library of MedicineView source
  4. Srygley FD, Gerardo CJ, Tran T, Fisher DA. Does this patient have a severe upper gastrointestinal bleed?. JAMA (2012)View source
  5. Mercado MG, Smith DK, Guard EL. Acute Kidney Injury: Diagnosis and Management. American Family Physician (2019)
  6. Evaluation of the Renal Patient: Blood Urea Nitrogen. Merck Manual Professional Version
  7. Clark WF, Sontrop JM, Huang SH, et al.. Effect of Coaching to Increase Water Intake on Kidney Function Decline in Adults With Chronic Kidney Disease: The CKD WIT Randomized Clinical Trial. JAMA (2018)View source
  8. Kreider RB, Kalman DS, Antonio J, et al.. International Society of Sports Nutrition position stand: safety and efficacy of creatine supplementation in exercise, sport, and medicine. Journal of the International Society of Sports Nutrition (2017)View source
  9. Rebholz CM, Crews DC, Grams ME, et al.. DASH (Dietary Approaches to Stop Hypertension) Diet and Risk of Subsequent Kidney Disease. American Journal of Kidney Diseases (2016)
  10. Kidney Failure Risk and Tests: BUN, Creatinine and GFR. National Kidney FoundationView source

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This page is educational and is not medical advice or a diagnosis. Lab results have to be interpreted together with your history, symptoms and other results by a clinician who knows you. Do not start or stop a supplement to change a lab value before the cause is known.