BUN · high result
High BUN: what it means, causes and how to read it with creatinine
Short answer
BUN (blood urea nitrogen) is the waste product of protein breakdown, cleared by the kidneys, so a high value — above about 20–25 mg/dL — can mean reduced kidney function but just as often means something else: dehydration, a high-protein diet or protein supplements, bleeding into the gut (digested blood is protein), heart failure, corticosteroids, or a catabolic state after illness or surgery. It is read with creatinine: when both are high the kidneys are the likely cause; when BUN is high and creatinine normal — a BUN-to-creatinine ratio above about 20 — dehydration, diet, bleeding or steroids are. An isolated mildly high BUN in someone who was fasting or dry is usually repeated hydrated before anything else.
Medically reviewed by Youmna DiStefano, MD · Last reviewed November 2026
When does this result need same-day care?
- A high BUN with black or bloody stools or vomiting blood — emergency care
- A high BUN with a rising creatinine, very little urine, confusion or severe swelling — same-day care
- A high BUN in someone unable to keep fluids down for more than a day, or with dizziness on standing — same-day care for dehydration
What is the normal range for BUN?
- age 18+: 7.00 – 20.0 mg/dL
Often reflects dehydration, high-protein diets, or reduced kidney function.
Full reference table, citations and testing options: BUN biomarker page.
What are the common causes of high BUN?
- Dehydration — poor fluid intake, vomiting, diarrhea, diuretics, a long fast before the draw; the kidneys reabsorb more urea when volume is low
- A high-protein diet, protein shakes and supplements, or a large meat meal the day before
- Reduced kidney function — acute kidney injury or chronic kidney disease; creatinine and eGFR move with it
- Gastrointestinal bleeding — blood in the gut is digested like a protein meal and raises BUN with a normal creatinine
- Heart failure and other low-flow states — less blood reaches the kidneys
- Corticosteroids (prednisone), tetracyclines, and catabolic states — fever, infection, surgery, burns, prolonged illness
- Urinary obstruction — an enlarged prostate, stones; usually with creatinine also rising
- Older age — a modest rise is common as kidney function declines slowly
What symptoms go with high BUN?
- Usually none from the BUN itself
- Thirst, dark urine and lightheadedness when dehydrated
- Fatigue, swelling, reduced urine output, nausea and itching when kidney function is significantly reduced
- Black or bloody stools when the cause is gut bleeding
What do clinicians usually check next?
- Read it with creatinine and eGFR on the same panel — the BUN-to-creatinine ratio sorts most cases (above about 20 points away from the kidneys)
- Repeat well hydrated and after a normal, not high-protein, day
- Review protein supplements, steroids and diuretics
- A stool blood test when the ratio is high without an obvious reason, and a CBC for anemia
- Urinalysis and a urine albumin-to-creatinine ratio when creatinine or eGFR is also abnormal
- Blood pressure, glucose and A1c, since diabetes and hypertension are the main drivers of kidney disease
- Nephrology referral when eGFR is persistently below 60 with a high BUN, or when kidney function is declining
Follow-up tests available at TestWell: CBC with Differential Test ($17.99), Glucose Test ($14.99).
How do you recheck BUN?
Prices are cash-pay; every order adds one $6 lab processing fee. Physician authorization is included; no insurance or referral needed.
| Test or panel | Type | Price | Fasting | Results |
|---|---|---|---|---|
| Basic Metabolic Panel | Test · 8 biomarkers | $17.99 | Yes | 1–3 business days |
Common questions
Is high BUN a kidney problem?
Only when creatinine and eGFR agree. On its own, high BUN is more often dehydration or diet.
What is a high BUN level?
Above the lab's upper limit, usually 20–25 mg/dL (7–8 mmol/L in international units). 25–40 is mild and usually dehydration, diet or steroids; above 40–50 with a raised creatinine suggests significant kidney impairment; above 100 is severe.
What does a high BUN with a normal creatinine mean?
That the kidneys are probably fine and something else is raising urea: dehydration, a high-protein diet, gut bleeding, steroids or heart failure. The BUN-to-creatinine ratio above 20 is the pattern, and the fix is usually fluids and a repeat.
Can dehydration raise BUN?
Yes — it is the most common cause. When blood volume is low the kidneys reabsorb more urea, so BUN climbs while creatinine stays near normal. A morning without fluids or a hot workout before the draw can push a normal person over the range.
Does a high-protein diet raise BUN?
Yes, modestly. Urea is the end product of protein metabolism, so protein shakes, a carnivore or high-protein diet, or a large steak the day before raise BUN with a normal creatinine. It is not harmful in itself with normal kidney function, but it should be known when the result is interpreted.
How do you lower a high BUN?
By addressing the cause: fluids if dehydrated, moderating protein intake and supplements, treating gut bleeding or heart failure, adjusting steroids. When kidney disease is the cause, the focus is on slowing it — blood pressure, glucose, avoiding NSAIDs — not on the BUN number itself.
Educational content, not a diagnosis. Lab results are one input; a clinician interprets them with your history and exam.