Lab result explainedKidneyLast updated November 8, 2026

BUN blood test: what high and low blood urea nitrogen levels mean

Blood urea nitrogen (BUN) · Basic or comprehensive metabolic panel (BMP/CMP), renal panel · reported in mg/dL (urea in mmol/L outside the US; BUN mg/dL × 0.357 = urea mmol/L)

Blood urea nitrogen (BUN) measures the amount of nitrogen in your blood that comes from urea, the waste product your liver makes when it breaks down protein. Your kidneys filter urea out into urine, so BUN is one of the two standard blood measures of kidney function, alongside creatinine. A normal BUN is about 7 to 20 mg/dL in adults (some labs use 6 to 24), and it rises when the kidneys are not clearing urea well, when you are dehydrated, or when there is more protein being broken down than usual.

BUN is less specific than creatinine. It goes up with dehydration, a high-protein meal or supplement, bleeding into the gut, steroids, heart failure and fever, none of which mean kidney damage, which is why a mildly high BUN with a normal creatinine is usually a hydration or diet question rather than a kidney one. The BUN-to-creatinine ratio helps: a ratio above 20:1 points toward dehydration or bleeding, while a ratio of 10 to 20 with both values raised points to the kidneys themselves.

A low BUN is less common and rarely serious. It usually reflects a low-protein diet, overhydration, pregnancy or a liver that is making less urea, as in advanced liver disease. Very high values, above about 100 mg/dL, usually mean advanced kidney failure and are often accompanied by the symptoms of uremia: nausea, loss of appetite, itching, confusion and fatigue.

BUN at a glance

  • What it is: the nitrogen content of urea in blood, a protein waste product made by the liver and cleared by the kidneys; a measure of kidney function, hydration and protein turnover.
  • Normal: about 7–20 mg/dL in adults (labs vary, 6–24 is also common); children 5–18 mg/dL; slightly higher after 60; lower in pregnancy. Outside the US urea is reported in mmol/L (BUN mg/dL × 0.357).
  • High: dehydration, high protein intake, bleeding in the gut, heart failure, steroids, urinary blockage or kidney disease. Above 100 mg/dL usually means advanced kidney failure.
  • Low: low-protein diet, overhydration, pregnancy, liver disease or malnutrition; rarely a problem in itself.
  • BUN/creatinine ratio over 20:1 suggests dehydration or bleeding rather than kidney damage; 10–20:1 with both raised suggests kidney disease.
  • Next step: read with creatinine, eGFR and electrolytes; a rising BUN with a rising creatinine, reduced urine output or swelling needs prompt assessment.

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What BUN measures

When your body breaks down protein, from food or from your own tissues, the amino acids release ammonia, which is toxic. The liver converts ammonia into urea through the urea cycle, and urea travels in the blood to the kidneys, where it is filtered freely by the glomeruli. About 40 to 50 percent of the filtered urea is reabsorbed back into the blood in the tubules, and this reabsorption increases when urine flow is slow, as it is in dehydration or low blood pressure. That is why BUN climbs more than creatinine when you are dry: less is filtered and more of what is filtered is taken back.

The test measures the nitrogen portion of urea in a blood sample using an enzyme (urease) reaction. In the US the result is reported as BUN in mg/dL; most other countries report the whole urea molecule in mmol/L. To convert, multiply BUN in mg/dL by 0.357 to get urea in mmol/L (a BUN of 20 mg/dL is a urea of about 7.1 mmol/L), or multiply BUN by 2.14 to get urea in mg/dL.

BUN depends on three things: how much urea is being made (protein intake, tissue breakdown, liver function), how well the kidneys filter it, and how much the tubules reabsorb (hydration and blood flow). Creatinine depends almost only on muscle mass and kidney filtration, which is why the two are read together and why eGFR is calculated from creatinine rather than BUN. BUN is not strictly a fasting test, but a large protein meal the night before can raise it by a few mg/dL.

Normal range for BUN

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

GroupTypical range (mg/dL (urea in mmol/L outside the US; BUN mg/dL × 0.357 = urea mmol/L))Notes
Adults, normal7–20 mg/dLMost US labs; some quote 6–24 or 8–23 mg/dL. Equivalent to urea of about 2.5–7.1 mmol/L. Labs differ; use the printed range.
Children (1–17 years)5–18 mg/dLLower because of smaller muscle mass and higher growth-related protein use.
Adults over 608–23 mg/dLKidney filtration declines gradually with age; values in the low 20s are common and are judged against creatinine and eGFR.
PregnancyAbout 3–13 mg/dLBlood volume and kidney filtration both rise, so BUN falls by roughly a quarter to a third; a 'normal' 18 mg/dL in late pregnancy deserves a second look.
Mildly raised21–40 mg/dLDehydration, high-protein diet, mild kidney impairment, heart failure, steroids, gut bleeding. Read with creatinine and the BUN/creatinine ratio.
Moderately raised41–100 mg/dLAcute kidney injury, moderate to advanced chronic kidney disease, severe dehydration, major gastrointestinal bleeding or urinary obstruction.
Severely raisedAbove 100 mg/dLAdvanced kidney failure; uremic symptoms usually present. Values above 150–200 mg/dL, or any level with confusion, pericarditis or severe nausea, are among the triggers for dialysis.
LowBelow 7 mg/dL (below 6 on some labs)Low protein intake, overhydration, pregnancy, liver disease or malnutrition. Rarely important on its own.

High BUN

A high BUN means urea is building up in your blood faster than your kidneys are clearing it. The most common reason in people who feel well is dehydration: with less fluid passing through the kidneys, urea is filtered more slowly and more of it is reabsorbed, so BUN rises while creatinine stays near normal. A high-protein diet or protein supplements, a recent large steak dinner, and medications such as corticosteroids and tetracyclines raise urea production in the same way. In these situations BUN is typically 21 to 35 mg/dL, the BUN/creatinine ratio is above 20:1, and the value falls back within a day or two of drinking more or changing the diet.

When BUN and creatinine rise together and the ratio stays around 10 to 20:1, the kidneys themselves are the problem: acute kidney injury from illness, low blood pressure or drugs (NSAIDs, some antibiotics, contrast dye), or chronic kidney disease from diabetes, high blood pressure or glomerular disease. Obstruction to urine flow from an enlarged prostate, kidney stones or a tumour also raises both. The level of BUN roughly tracks severity: in stage 3 chronic kidney disease BUN is often 25 to 40, in stage 4 it is 40 to 80, and in kidney failure it exceeds 100 mg/dL.

Two situations deserve particular mention. Bleeding into the stomach or upper gut is digested like a protein meal and can raise BUN sharply (often above 40) with a normal creatinine, so a high ratio with black stools or a falling hemoglobin points to a bleed. And in heart failure, a raised BUN reflects poor kidney blood flow and is one of the strongest predictors of a poor outcome, even more so than creatinine, which is why it is tracked closely in people with heart disease.

Common causes

  1. Dehydration

    Vomiting, diarrhoea, sweating, diuretics or simply not drinking enough reduce kidney blood flow and increase urea reabsorption; BUN often rises to 25–40 mg/dL with a BUN/creatinine ratio above 20:1 and falls within 24 to 48 hours of rehydration.

  2. Chronic kidney disease

    Diabetes, high blood pressure, glomerulonephritis and polycystic kidney disease progressively reduce filtration. BUN rises alongside creatinine as eGFR falls below about 60 mL/min, and exceeds 100 mg/dL in kidney failure.

  3. Acute kidney injury

    Sepsis, major surgery, low blood pressure, rhabdomyolysis, contrast dye, NSAIDs, ACE inhibitors in dehydrated people and some antibiotics can drop kidney function within days; BUN can climb 10 to 20 mg/dL per day in severe cases.

  4. High protein intake or protein supplements

    Each extra 10 g of dietary protein produces roughly 1.6 g of urea nitrogen; diets above about 1.5–2 g of protein per kg or large whey or amino-acid supplements commonly push BUN into the 20s with normal creatinine.

  5. Bleeding into the stomach or upper intestine

    Blood proteins are digested and absorbed; a BUN above 30–40 mg/dL with a normal creatinine and a ratio above 30:1 is a classic sign of an upper gastrointestinal bleed, especially with black tarry stools.

  6. Heart failure and low blood pressure

    Reduced cardiac output lowers kidney perfusion; BUN above about 40 mg/dL on admission for heart failure roughly doubles the risk of death compared with normal values and is used in risk scores.

  7. Urinary tract obstruction

    Enlarged prostate, kidney stones, bladder or pelvic tumours or a blocked catheter back urine up into the kidneys; BUN and creatinine both rise and fall quickly once the blockage is relieved.

  8. Corticosteroids, tetracyclines and other medications

    Steroids increase protein breakdown; tetracyclines inhibit protein synthesis and raise urea; diuretics cause dehydration; NSAIDs, lithium, some chemotherapy and high-dose intravenous contrast reduce kidney function.

  9. Increased tissue breakdown

    Fever, severe infection, major burns, trauma, surgery, prolonged fasting and uncontrolled diabetes all increase muscle protein breakdown and therefore urea production.

  10. Older age

    Kidney filtration falls by about 1 mL/min per year after 40, so BUN in the low 20s is common in people over 70 and is interpreted with eGFR rather than treated as abnormal on its own.

Symptoms that can go with it

  • Often none when BUN is only mildly raised; dehydration may cause thirst, dark urine, dizziness on standing and a dry mouth
  • Fatigue, weakness and difficulty concentrating as urea climbs
  • Loss of appetite, nausea, vomiting and a metallic or ammonia-like taste with advanced kidney disease (uremia)
  • Itching all over without a rash
  • Swelling of the ankles, legs, hands or around the eyes
  • Passing less urine, or passing urine more often at night
  • Shortness of breath from fluid build-up
  • Confusion, muscle twitching or, rarely, seizures at very high levels

Low BUN

A low BUN means less urea is being made or it is being diluted. The most common explanation in a healthy person is a diet low in protein, as in some vegan or restrictive eating patterns, or drinking a lot of fluid, which expands blood volume and speeds urea clearance. BUN also falls by about a quarter to a third in pregnancy because blood volume and kidney filtration rise, so a value of 4 to 8 mg/dL in a pregnant woman is normal.

Because the liver makes urea, a low BUN can also reflect reduced liver function. In advanced cirrhosis or acute liver failure the liver cannot convert ammonia to urea efficiently, so BUN falls while ammonia rises; a low BUN alongside low albumin, a raised bilirubin or a prolonged clotting time fits this picture. Severe malnutrition, with little protein coming in and muscle wasting, lowers BUN too, and in that setting a low value is a marker of poor nutritional status rather than good kidney health.

Low BUN is not harmful in itself and is rarely the reason for further tests. It is followed up when it comes with signs of liver disease, when it is very low (under 3 to 4 mg/dL) in a child, which can hint at a rare inherited urea cycle disorder, or when it accompanies the low sodium of overhydration or SIADH.

Common causes

  1. Low-protein diet

    Urea production tracks protein intake; diets providing under about 0.6–0.8 g of protein per kg, including some plant-based or very restricted diets, commonly give BUN values of 4 to 7 mg/dL.

  2. Overhydration

    Drinking very large volumes, intravenous fluids, or water retention from SIADH or psychogenic polydipsia dilute urea and lower BUN, usually alongside a low sodium.

  3. Pregnancy

    Plasma volume rises by about 40 percent and kidney filtration by about 50 percent, so BUN typically runs 3 to 13 mg/dL, lowest in the second trimester.

  4. Liver disease

    Cirrhosis, alcoholic hepatitis and acute liver failure impair the urea cycle; BUN falls while blood ammonia rises, and a low BUN with low albumin and raised bilirubin reflects reduced liver synthetic function.

  5. Malnutrition and muscle wasting

    Prolonged inadequate intake, anorexia nervosa, cancer cachexia and advanced age with sarcopenia reduce both protein intake and the muscle pool that generates urea.

  6. Anabolic states and medications

    Growth hormone, anabolic steroids and the rapid growth of childhood divert amino acids into tissue rather than urea; a low BUN in a growing child is common and normal.

  7. Urea cycle disorders

    Rare inherited enzyme deficiencies (for example ornithine transcarbamylase deficiency) prevent urea formation; a persistently very low BUN with high ammonia, vomiting and lethargy, usually in infancy or childhood, prompts testing.

Symptoms that can go with it

  • Usually none; a low BUN from diet, fluids or pregnancy causes no symptoms
  • Fatigue, muscle loss, hair thinning or slow wound healing when caused by malnutrition
  • Jaundice, abdominal swelling, easy bruising or confusion when caused by liver disease
  • Headache, nausea or confusion with the low sodium of overhydration
  • In infants and children, poor feeding, vomiting, lethargy or irritability with a very low BUN may suggest a urea cycle disorder

Read it alongside

  • Creatinine — the more specific kidney marker; BUN high with normal creatinine points to dehydration, diet or bleeding, while both high points to the kidneys.
  • BUN/creatinine ratio — above 20:1 suggests dehydration, gut bleeding, steroids or heart failure; 10–20:1 suggests intrinsic kidney disease; under 10:1 suggests liver disease, low protein intake or rhabdomyolysis.
  • eGFR — the standard estimate of kidney filtration, calculated from creatinine; it stages chronic kidney disease, which BUN alone cannot.
  • Sodium and potassium — low sodium with low BUN suggests overhydration or SIADH; high potassium with high BUN and creatinine signals kidney failure that needs urgent attention.
  • CO2 (bicarbonate) and anion gap — fall and rise respectively as kidney failure causes acid to accumulate.
  • Hemoglobin and hematocrit — a falling hemoglobin with a disproportionately high BUN suggests gastrointestinal bleeding; a high hematocrit supports dehydration.
  • Albumin, bilirubin and liver enzymes — a low BUN with low albumin and raised bilirubin points to reduced liver synthetic function.
  • Urinalysis and urine albumin — protein or blood in the urine with a raised BUN indicates kidney disease rather than a prerenal cause.

What usually happens next

  1. A mildly high BUN (21–35 mg/dL) with a normal creatinine is usually repeated after a few days of normal fluid intake, without protein supplements the day before; most return to range.
  2. If creatinine is also raised, expect an eGFR, urinalysis, urine albumin-to-creatinine ratio, electrolytes and a review of medications (NSAIDs, diuretics, blood pressure drugs, metformin dosing) to establish whether this is acute or chronic kidney disease and how advanced.
  3. A high BUN with a normal creatinine and black stools, a drop in hemoglobin or dizziness needs same-day assessment for gastrointestinal bleeding.
  4. In known kidney disease, BUN is tracked alongside creatinine, potassium, bicarbonate and phosphorus; the trend over months matters more than any single value.
  5. A low BUN is interpreted with albumin, liver tests and sodium; if these are normal and your diet is low in protein, nothing further is usually needed.
  6. Ask what your BUN/creatinine ratio is and whether your result was drawn after a high-protein meal, strenuous exercise or dehydration, all of which can be corrected before a repeat.

Diet, lifestyle and natural levers with evidence

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

Adequate hydration

Moderate evidence

Dehydration is the most common reversible cause of a raised BUN. Studies of fluid restriction in healthy adults show BUN rising by several mg/dL within 24 to 36 hours and returning to baseline within a day of rehydration, and in older adults in hospital a BUN/creatinine ratio above 20:1 is used as a marker of dehydration. For most people, drinking enough that urine stays pale yellow (roughly 2 to 3 litres of total fluid a day in temperate conditions) keeps BUN at baseline.

People with heart failure, advanced kidney disease or on fluid restriction should follow their clinician's fluid targets; more is not better in those conditions.

Moderate rather than very high protein intake

Strong evidence

BUN is directly proportional to protein intake. In controlled feeding studies, moving from about 0.8 to 1.5–2 g of protein per kg per day raises BUN by roughly 5 to 10 mg/dL in people with normal kidneys, and stopping large protein or amino-acid supplements lowers it within days. In chronic kidney disease, a Cochrane review of 17 trials found that low-protein diets (0.5–0.8 g/kg/day) reduce urea and slow progression to kidney failure, with very-low-protein diets lowering the risk of reaching dialysis by about a third in stage 4–5 disease.

Protein restriction in kidney disease must be supervised by a renal dietitian to avoid malnutrition; older adults without kidney disease generally need 1.0–1.2 g/kg/day and should not cut protein to lower a mildly raised BUN.

Dietary fibre, particularly from whole grains, legumes, fruit and vegetables

Moderate evidence

Fibre feeds gut bacteria that use nitrogen for their own growth, diverting urea into stool rather than blood. A meta-analysis of 14 controlled feeding trials in people with chronic kidney disease found that adding dietary fibre (median about 27 g a day) lowered serum urea by about 1.8 mmol/L, equivalent to a BUN drop of roughly 5 mg/dL, and lowered creatinine modestly. Plant-based eating patterns are also associated with slower kidney decline in cohort studies.

In advanced kidney disease, potassium and phosphorus from some high-fibre foods need to be balanced with a dietitian's guidance.

Avoid NSAIDs and keep blood pressure and blood sugar at target

Strong evidence

Ibuprofen, naproxen and similar painkillers reduce blood flow through the kidneys and can raise BUN and creatinine within days, particularly in older adults, in dehydration and alongside diuretics or ACE inhibitors; stopping them often reverses the rise. Over years, controlling blood pressure (below 130/80 mmHg in most guidelines) and A1C (around 7 percent) roughly halves the rate of kidney function loss in people with diabetes or hypertension, which is what prevents BUN from climbing in the first place.

Limit alcohol and treat heart failure and liver disease causes

Moderate evidence

Alcohol dehydrates, injures the liver and, in heavy use, contributes to heart failure, all of which disturb BUN in different directions. In heart failure, achieving a stable weight on appropriate treatment lowers BUN as kidney blood flow improves, and a falling BUN during treatment of decompensated heart failure predicts a better outcome. Where a low BUN reflects cirrhosis, abstinence is the single most effective measure for recovering liver function and urea production.

When to see a clinician promptly

  • A BUN above 100 mg/dL, or above 60 with nausea, vomiting, confusion, chest pain or severe shortness of breath: possible advanced kidney failure and uremia, needs same-day assessment.
  • A rising BUN and creatinine over days with reduced urine output, swelling or breathlessness: possible acute kidney injury.
  • A high BUN with black, tarry or bloody stools, vomiting blood, light-headedness or a falling hemoglobin: possible gastrointestinal bleeding, an emergency.
  • A high BUN with potassium above 6.0 mmol/L, which can cause dangerous heart rhythms, or with bicarbonate below 15 mEq/L.
  • A high BUN alongside inability to pass urine, severe flank pain or a very full bladder: possible obstruction.
  • A persistently high BUN or BUN/creatinine ratio in someone with heart failure, diabetes or known kidney disease, to review medications and fluid balance.

Frequently asked questions

What is a normal BUN level?

For adults, about 7 to 20 mg/dL on most US labs, with some using 6 to 24 mg/dL. Children run 5 to 18 mg/dL, people over 60 are often a little higher (up to about 23), and pregnant women lower (3 to 13). Outside the US, urea is reported in mmol/L; a BUN of 7–20 mg/dL is a urea of roughly 2.5–7.1 mmol/L. Always compare with the range printed on your own report.

Is a BUN of 22, 25 or 30 high?

Slightly. A BUN of 22 to 30 mg/dL is mildly raised, and if your creatinine and eGFR are normal the usual explanations are dehydration, a high-protein diet or supplement, a recent large meat meal, steroids or a diuretic. It generally returns to range on a repeat after a couple of days of normal fluid intake. If creatinine is also raised, or the BUN keeps rising, kidney function is assessed properly with eGFR and a urine test.

What causes high BUN levels?

Dehydration is the most common cause, followed by high protein intake, kidney disease (acute or chronic), heart failure, bleeding into the stomach or gut, urinary blockage from an enlarged prostate or stones, and medications such as corticosteroids, NSAIDs, tetracyclines and diuretics. Fever, burns, surgery and other states of tissue breakdown also push it up. The BUN/creatinine ratio and the trend over time help sort out which.

Can dehydration raise BUN?

Yes, and it is the most frequent reason for a mildly high result. When you are dry, less blood flows through the kidneys and more urea is reabsorbed from the filtered urine, so BUN rises, often to 25–40 mg/dL, while creatinine changes little. A BUN/creatinine ratio above 20:1 supports dehydration. Rehydrating usually normalises BUN within 24 to 48 hours.

What does a high BUN with a normal creatinine mean?

It means the kidneys are probably filtering normally but something is raising urea production or reabsorption: dehydration, a high-protein diet, corticosteroids, heart failure or bleeding into the gut. The ratio of BUN to creatinine is typically above 20:1 in these situations. A gastrointestinal bleed is the one to rule out promptly if there are black stools, dizziness or a drop in hemoglobin.

What is the BUN/creatinine ratio and what is normal?

It is BUN divided by creatinine, both in mg/dL, and is normally about 10 to 20:1 (for example BUN 15 and creatinine 1.0 gives 15:1). Above 20:1 suggests dehydration, gut bleeding, steroids, heart failure or a high-protein diet; below 10:1 suggests liver disease, low protein intake, overhydration or rhabdomyolysis. The ratio is a guide, not a diagnosis, and is less reliable in people with very low muscle mass.

What BUN level requires dialysis?

There is no single number. Dialysis is started for the consequences of kidney failure (uremic symptoms such as persistent nausea, confusion or pericarditis, fluid overload that does not respond to diuretics, dangerous potassium or acid levels), which usually occur when BUN is above about 100 mg/dL and eGFR is below 10–15 mL/min. Some people tolerate a BUN of 120 with few symptoms, while others are unwell at 80. The decision is clinical, not based on BUN alone.

How can I lower my BUN?

If kidney function is normal, drinking enough fluid, moderating protein intake (stopping large protein or amino-acid supplements), eating more fibre-rich plant foods and avoiding NSAIDs typically brings a mildly high BUN back to range within days. If BUN is high because of kidney disease, the levers are controlling blood pressure and blood sugar, reviewing medications, and a dietitian-guided protein plan; these slow the rise but the underlying kidney function determines the level.

References

  1. BUN (blood urea nitrogen) test. MedlinePlus, US National Library of Medicine (2023)View source
  2. Blood urea nitrogen (BUN) test. Mayo Clinic (2024)View source
  3. Understanding your lab values and other CKD health numbers. National Kidney Foundation (2024)
  4. Hosten AO. BUN and creatinine. Clinical Methods: The History, Physical, and Laboratory Examinations, 3rd edition (1990)
  5. Rahman M, Shad F, Smith MC. Acute kidney injury: a guide to diagnosis and management. American Family Physician (2012)
  6. Kidney Disease: Improving Global Outcomes (KDIGO) Acute Kidney Injury Work Group. KDIGO clinical practice guideline for acute kidney injury. Kidney International Supplements (2012)
  7. Aronson D, Mittleman MA, Burger AJ. Elevated blood urea nitrogen level as a predictor of mortality in patients admitted for decompensated heart failure. American Journal of Medicine (2004)
  8. Ernst AA, Haynes ML, Nick TG, Weiss SJ. Usefulness of the blood urea nitrogen/creatinine ratio in gastrointestinal bleeding. American Journal of Emergency Medicine (1999)
  9. Chiavaroli L, Mirrahimi A, Sievenpiper JL, Jenkins DJ, Darling PB. Dietary fiber effects in chronic kidney disease: a systematic review and meta-analysis of controlled feeding trials. European Journal of Clinical Nutrition (2015)
  10. Hahn D, Hodson EM, Fouque D. Low protein diets for non-diabetic adults with chronic kidney disease. Cochrane Database of Systematic Reviews (2020)
  11. Evaluation of the renal patient. Merck Manual Professional Version (2024)

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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.