Blood Urea Nitrogen (BUN) Test: Normal Levels, High & Low

BUN stands for blood urea nitrogen — the amount of nitrogen carried in your blood as urea, a waste product your liver makes when it breaks down protein and your kidneys filter out into urine. On a blood test, BUN is almost always reported next to creatinine, which is why the BUN/creatinine ratio comes up so often. But BUN is a less specific measure of kidney function than creatinine: it swings widely with hydration and diet. This guide explains what BUN means on a blood test, normal BUN levels, what causes a high BUN or a low BUN, and how to read the BUN/creatinine ratio.

Key takeaways

• BUN measures your body's main nitrogen waste: urea is produced by the liver from protein and cleared by the kidneys.¹
• It partly reflects kidney function, but less reliably than creatinine and eGFR, because it also moves with hydration, diet, and other factors.²
• Typical normal BUN levels run about 7–20 mg/dL in adults, with slightly different ranges by age and sex — and by lab.³
• A high BUN most often comes from dehydration, a high-protein diet, gastrointestinal bleeding, or kidney disease.⁴
• A low BUN is usually benign (low-protein diet, liver disease, pregnancy, overhydration) and rarely needs treatment.⁵
• The BUN/creatinine ratio (normally about 10:1 to 20:1) helps separate dehydration from intrinsic kidney injury — but it is imperfect and never read alone.⁶

What is blood urea nitrogen (BUN)?

When your body uses protein — from food or from its own tissues — it produces ammonia, which is toxic. The liver converts that ammonia into urea, a far less toxic molecule. Urea then travels in the blood to the kidneys, which filter it out into the urine. Urea is the largest circulating "reservoir" of nitrogen in the body, which is why the test measures the nitrogen portion of urea rather than urea itself.¹

Because the kidneys clear it, BUN rises when they filter less effectively. But BUN is less specific than creatinine: its concentration also depends heavily on how much protein you eat, how hydrated you are, and how much urea your kidney tubules reabsorb back into the blood.² That is why BUN is interpreted alongside creatinine and eGFR, not on its own.

BUN is rarely ordered by itself. It comes bundled into a basic metabolic panel (BMP) or a comprehensive metabolic panel (CMP) — routine panels that also report glucose, electrolytes, calcium, and creatinine, and, in the CMP, liver markers.⁷ Your primary care provider (PCP) typically orders one at a checkup or when investigating symptoms; these panels are standard, widely covered by insurance, and require no special preparation in most cases.

Why is BUN measured?

Clinicians check BUN to:⁵

• assess kidney function, as a complement to creatinine and eGFR;
• gauge hydration status (dehydration raises it);
• help work up acute kidney injury, using the BUN/creatinine ratio;
• monitor specific situations — gastrointestinal bleeding, malnutrition, heart failure, and dialysis adequacy.

Normal BUN levels

Below are typical reference ranges for adults and children. They vary by laboratory, age, and sex — and BUN normally increases with age — so always compare your number to the range printed on your report.⁵

Values by age and sex per Cleveland Clinic.³

Good to know: in the United States, BUN is reported in mg/dL. Many other countries (including France) instead report urea in mmol/L or g/L — a different quantity, not just a different unit. To convert: urea (mmol/L) × 2.8 = BUN (mg/dL), and BUN (mg/dL) = urea (mg/dL) ÷ 2.14. So a European urea of 2.5–7.5 mmol/L corresponds to a BUN of roughly 7–21 mg/dL — the same range in different clothing. BUN also swings a lot with diet and hydration: a single, mildly elevated result is often unremarkable.

What a high BUN means

A high BUN does not automatically mean kidney disease. Common causes include:²⁴

• dehydration or low blood flow to the kidneys (the kidney reabsorbs more water — and more urea) — a very frequent cause;
• a high-protein diet, or protein supplements;
• gastrointestinal bleeding, where digested blood delivers a large protein load and drives BUN up (roughly 500 mL of blood in the gut is equivalent to about 100 g of protein);⁶⁴
• increased catabolism — infection, corticosteroids, fasting, burns, major stress;
• kidney disease (BUN rises together with creatinine), heart failure, or a urinary blockage;
• certain medications, including diuretics and some antibiotics.³

To sort this out, your clinician looks at creatinine and the BUN/creatinine ratio: a high ratio points more toward a "prerenal" cause (dehydration, GI bleeding, poor perfusion), while a ratio nearer normal, with both values rising together, points toward intrinsic kidney injury. This ratio is imperfect and is never sufficient on its own.²

What a low BUN means

A low BUN — below roughly 6–7 mg/dL — is far less discussed than a high one, but it is a common search and a common result. It is usually not a cause for concern and rarely requires treatment.⁵ The main explanations:

• Low protein intake or malnutrition. BUN is downstream of dietary protein. A vegetarian or vegan diet, a deliberately low-protein diet, an eating disorder, or simple undernutrition all reduce urea production and drag BUN down.⁸ Here the low number is a nutritional signal, not a kidney one.
• Liver disease. The liver is the factory: it converts ammonia into urea. In advanced liver disease — cirrhosis, severe hepatitis, liver failure — the liver synthesizes less urea, so BUN falls even if the kidneys are perfectly fine. A low BUN alongside abnormal liver tests is the pattern worth flagging.¹
• Overhydration. Excess fluid — from heavy water intake, IV fluids, or SIADH — dilutes urea in the blood and lowers the measured BUN without any change in production or clearance.³
• Pregnancy. Blood volume expands and kidney filtration increases during pregnancy, so BUN normally runs lower than usual. This is expected physiology, not a problem.
• Small body size, and, in some cases, anabolic states where nitrogen is being built into tissue rather than excreted.³

A low BUN paired with a low BUN/creatinine ratio especially suggests inadequate protein intake, reduced urea synthesis from advanced liver disease, or more efficient urea removal — for example during dialysis.⁶ As with a high result, the number matters only in context: an isolated low BUN in someone eating a light or plant-forward diet is generally meaningless on its own.

The BUN/creatinine ratio

The BUN/creatinine ratio is simply your BUN (mg/dL) divided by your creatinine (mg/dL). Because both are unitless once divided, the result is expressed as a ratio — normally about 10:1 to 20:1.⁶ It is a staple of American lab interpretation, and it exists because BUN and creatinine are handled differently by the kidney: creatinine is filtered and essentially not reabsorbed, while urea is partly reabsorbed — and reabsorbed more when the kidney is trying to conserve water.

That difference is what makes the ratio informative:

• Ratio above ~20:1 — suggests a prerenal cause: dehydration, blood loss, heart failure, or any state of low kidney perfusion, where the kidney avidly reabsorbs water and urea while creatinine keeps being excreted. Gastrointestinal bleeding is the classic non-perfusion cause of a high ratio, since digested blood floods the system with protein.⁴
• Ratio around 10:1–20:1 with both values rising — points toward intrinsic kidney disease, where filtration falls for both molecules roughly in step.
• Ratio below ~10:1 — suggests low protein intake, advanced liver disease (less urea made), rhabdomyolysis (creatinine rises disproportionately), or dialysis.⁶

Two important caveats. First, the 10:1 baseline holds best in moderate to advanced renal failure; the "normal" reference point shifts with the degree of kidney dysfunction present.⁶ Second, because BUN answers to so many non-kidney inputs — diet, hydration, tubular reabsorption, liver function — the ratio is useful but imperfect at separating prerenal from intrinsic causes. It orients the work-up; it never closes it.²

BUN as a marker of severity

Beyond the kidney, a high BUN is also a prognostic marker in certain acute illnesses — notably heart failure and critical illness — where it captures hydration, perfusion, and catabolism all at once.⁹ The BUN/creatinine ratio carries prognostic weight too: it has been studied in cardiogenic shock and in intensive care populations, where a higher admission ratio tracks with worse outcomes.¹⁰ This remains a clinical reading performed by physicians, never a diagnosis in itself.

What can affect your BUN

BUN depends on protein intake and nutritional status,⁸ hydration, kidney function, the liver (production), age (it rises over the lifespan), certain medications (diuretics, corticosteroids, some antibiotics), and any gastrointestinal bleeding.³ This is exactly what makes it less specific than creatinine for judging the kidneys — and why formal assessment of kidney function rests on estimated GFR and albuminuria rather than BUN.¹¹¹²

Recent research

According to recent PubMed publications:

• BUN vs. creatinine. Reviews reiterate that urea is a less specific marker of kidney function than creatinine and eGFR, because it is strongly influenced by diet, medications, and tubular reabsorption.²
• The BUN/creatinine ratio, qualified. Since BUN answers to so many variables (diet, hydration, reabsorption), the ratio is useful but imperfect for distinguishing a prerenal cause from intrinsic kidney injury; it should be read cautiously and never alone.²⁶
• Prognostic value. A high BUN — or a high BUN/creatinine ratio — on admission is associated with worse outcomes in acute decompensated heart failure and other severe states; the ratio has also been studied in cardiogenic shock and critical care.⁹¹⁰
• For judging the kidneys, it's eGFR. The KDIGO 2024 guideline bases evaluation and staging of chronic kidney disease on estimated GFR and albuminuria — not on BUN.¹³¹² U.S. practice follows the same logic: NIDDK's laboratory guidance centers on eGFR reporting and urine albumin.¹¹
• BUN, protein, and nutrition. BUN partly reflects protein intake and nutritional/muscle status — a factor to weigh in interpretation alongside creatinine.⁸

These findings concern interpretation and prognosis; they do not justify self-medication and do not replace your physician's advice.

Get your BUN interpreted by AI DiagMe

A BUN is never read alone: its meaning depends on your creatinine and eGFR, your hydration, your diet, and your wider context. That cross-referencing is what gives the result its real value.

👉 AI DiagMe interprets your lab results — blood, urine, or stool — in plain language, taking your whole profile into account. An informational service that does not provide a diagnosis and complements, never replaces, your physician.

Frequently asked questions

Bottom line

BUN (blood urea nitrogen) measures a protein waste product made by the liver and cleared by the kidneys. It partly reflects kidney function, but less reliably than creatinine, because it also tracks hydration and diet. Normal is roughly 7–20 mg/dL. A high BUN often signals dehydration, a high-protein diet, or GI bleeding as much as a kidney problem; a low BUN is usually benign, pointing to low protein intake, liver disease, pregnancy, or overhydration. The BUN/creatinine ratio (normally 10:1–20:1) helps orient but stays imperfect. No single value is read alone: what matters is the full set of your markers and your profile — which is what AI DiagMe provides, alongside your physician.

Sources

Official sources and peer-reviewed publications (PubMed) used for this guide: