Bedside Snapshot
- Normal (Adult)
- 8–23 mg/dL (urea 2.9–8.2 mmol/L)
Read with creatinine; lab-specific - Critical Values
- No universal critical value: interpret by trend and clinical context. Uremic signs need emergent dialysis evaluation at any BUN.
- High Suggests
- Shock, hypovolemia or heart failure
- Upper GI bleeding
- AKI, CKD or obstruction
- Catabolism, steroids or protein load
- Low Suggests
- Liver failure
- Malnutrition or low protein intake
- Overhydration
- Pregnancy (normal)
- Act Now If
- Rising BUN with confusion, pericardial rub, bleeding, high K⁺, acidosis or pulmonary edema: monitor, 12-lead ECG, recheck K⁺ and blood gas; urgent nephrology for dialysis
- Watch Out For
- High BUN with a normal creatinine is often not kidney disease: look for GI bleeding, dehydration or steroids. A normal BUN does not rule out a reduced GFR.
Reference & Critical Ranges
Lab-specific ranges: Every lab sets its own interval, and it shifts with the analyzer and the population. Trust the range on your report. The values below are typical, not a rule.
Reference ranges
| Population | Conventional | SI (urea) | Notes |
|---|---|---|---|
| Adult | 8–23 mg/dL | 2.9–8.2 mmol/L | Critical Care Transport interval; many US labs print 6–20 or 7–20 mg/dL |
| Older adult | 8–23 mg/dL | 2.9–8.2 mmol/L | Same interval in many labs, though BUN drifts up with age (28–35 mg/dL in an older adult is not alarming by itself); low muscle mass lowers creatinine, so BUN:Cr runs higher |
| Pregnancy, 1st trimester | 7–12 mg/dL | 2.5–4.3 mmol/L | Falls as GFR and plasma volume rise |
| Pregnancy, 2nd trimester | 3–13 mg/dL | 1.1–4.6 mmol/L | — |
| Pregnancy, 3rd trimester | 3–11 mg/dL | 1.1–3.9 mmol/L | An adult-normal 15–20 mg/dL is above the pregnancy range |
| Neonate and young infant (0–2 months) | 4–15 mg/dL | 1.4–5.4 mmol/L | Use age-specific intervals |
| Infant (2 months–2 years) | 5–15 mg/dL | 1.8–5.4 mmol/L | — |
| Child and adolescent (≥2 years) | 8–23 mg/dL | 2.9–8.2 mmol/L | Same as adult |
Severity & critical values
| Level | Conventional | SI (urea) | What it means |
|---|---|---|---|
| Low | <8 mg/dL | <2.9 mmol/L | Low urea production (liver failure, low protein intake, malnutrition) or dilution; rarely dangerous by itself (below the lab's lower limit; many labs print 6 or 7) |
| Normal | 8–23 mg/dL | 2.9–8.2 mmol/L | Does not exclude a reduced GFR; creatinine, eGFR and urine output are better kidney markers |
| Elevated (azotemia) | >23 mg/dL | >8.2 mmol/L | Read with creatinine and BUN:Cr: low perfusion, GI bleeding, catabolism, AKI, CKD or obstruction (above the lab's upper limit: 23 mg/dL on the Critical Care Transport range, 20 mg/dL in many US labs) |
| Rising in AKI | 10–20 mg/dL per day | 3.6–7.1 mmol/L per day | Typical daily climb in established AKI; catabolism and GI blood add to it |
| Critical | No universal critical value: interpret by trend and clinical context | — | Typical; lab-specific. Uremic complications, refractory K⁺, acidosis or pulmonary edema need emergent dialysis evaluation at any BUN |
Critical values: There is no universal critical value (typical; lab-specific). Read the trend and the clinical context. Uremic encephalopathy, pericarditis or bleeding, refractory hyperkalemia, refractory acidosis or pulmonary edema need emergent dialysis evaluation at any BUN.
Conversion: BUN mg/dL × 0.357 = urea mmol/L; BUN mg/dL × 2.14 = urea mg/dL
BUN:creatinine ratio
BUN (mg/dL) ÷ creatinine (mg/dL)
Typical reading: a ratio above 20:1 points toward prerenal physiology or an added urea load. ATN often lands at 10–15:1 or lower, and the ranges overlap across causes. Treat the ratio as a clue, not a diagnosis. Conventional units only.
Causes of Abnormal Results
High (azotemia, BUN >23 mg/dL)
Azotemia is simply a high BUN, usually with a high creatinine too. Uremia is azotemia with symptoms or complications. Watch for anorexia, nausea, vomiting, confusion, asterixis, myoclonus, seizures, pericarditis and impaired coagulation.
| Mechanism | Common causes | Bedside clue |
|---|---|---|
| Low kidney perfusion (prerenal) | Shock, hemorrhage, vomiting, diarrhea, diuretics, heart failure, renal artery stenosis (especially bilateral, with an ACE inhibitor or ARB) | BUN:Cr >20:1, urine Na⁺ <10 mmol/L, fractional excretion of sodium (FENa) <1%, hypotension |
| Upper GI bleeding | Peptic ulcer, varices, bleeding on anticoagulants | Melena or hematemesis; BUN up out of proportion to creatinine; falling hemoglobin |
| Protein catabolism or load | Glucocorticoids, surgery, trauma, burns, parenteral nutrition, prolonged fasting, tetracyclines | Creatinine normal or near-normal; recent steroids, injury or feeding change |
| Intrinsic AKI | Acute tubular necrosis (ATN) from sepsis, hypotension or nephrotoxins (aminoglycosides, IV iodinated contrast, vancomycin, NSAIDs); glomerulonephritis; interstitial nephritis | Rising creatinine; BUN:Cr often 10–15:1 or lower in ATN, with large overlap; urine Na⁺ >40 mmol/L, FENa >1% |
| Postrenal obstruction | Prostatic enlargement, stones, bladder outlet obstruction, pelvic or retroperitoneal mass | Distended bladder, low or absent urine output; BUN:Cr usually not elevated |
| Chronic kidney disease (CKD) | Diabetic or hypertensive kidney disease, other chronic nephropathy | Stable elevation, known low eGFR; compare with baseline |
High BUN with a normal creatinine: Before you blame the kidneys, think upper GI bleeding, volume depletion or catabolism. Check hemoglobin, the stool, recent steroids or feeds, and volume status.
Low (BUN <8 mg/dL)
| Mechanism | Common causes | Bedside clue |
|---|---|---|
| Reduced urea synthesis | Liver failure, cirrhosis | Jaundice, coagulopathy, encephalopathy; BUN can be low even with normal kidneys |
| Low protein intake | Malnutrition, low-protein diet, protein-restricted advanced CKD | Poor intake or weight loss; in CKD stage 4–5 a low-protein diet lowers BUN and BUN:Cr |
| Dilution or higher GFR | Overhydration, pregnancy | Large IV volumes; pregnancy has its own lower ranges |
Interpretation & Bedside Approach
Step-by-step approach
- Is it real? Check the analyte and the unit: BUN or urea, where urea in mg/dL is 2.14 × BUN. Make sure the sample was not drawn above a running IV. Repeat a value that surprises you before you act on it alone.
- How sick is the patient? Look for shock, GI bleeding and uremic signs: confusion, asterixis, pericardial rub, bleeding. K⁺, bicarbonate and volume status set the urgency far more than the BUN does.
- Acute or chronic? Pull up the baseline and compare. Once AKI is established, BUN typically climbs 10–20 mg/dL per day. A steady elevation with a known low eGFR fits CKD. The trend tells you more than any one value.
- Calculate BUN:Cr. Above 20:1 supports prerenal physiology, GI bleeding or catabolism. A ratio below about 10–15:1 often shows up with tubular injury (ATN) or low urea production. A ratio above 30:1 with melena or hematemesis favors an upper GI source, but a lower ratio does not rule one out (see the BUN:creatinine ratio page). The causes overlap a lot, so use the ratio as support, never alone.
- Classify with named criteria. KDIGO 2012 defines AKI by creatinine: a rise of ≥0.3 mg/dL within 48 hr, or ≥1.5 × baseline within 7 days. Urine output below 0.5 mL/kg/hr for 6 hr does too. BUN is not one of the criteria, though several severity scores use it (table below).
- Escalate. Get emergent dialysis evaluation for life-threatening fluid, electrolyte or acid–base problems, or for uremic complications. KDIGO 2012 says to decide when to start from the clinical context and the trend, not from one BUN or creatinine threshold.
BUN and creatinine patterns
| Pattern | Suggests |
|---|---|
| High BUN, normal creatinine, BUN:Cr >20:1 | Early volume depletion, upper GI bleeding, steroids, catabolism or high protein intake |
| High BUN and creatinine, BUN:Cr >20:1 | Prerenal AKI (hypovolemia, heart failure, shock), or prerenal stress on CKD |
| High BUN and creatinine, BUN:Cr below about 10–15:1 | Often tubular injury (ATN); also low protein intake, liver disease or a creatinine surge (rhabdomyolysis) |
| High BUN and creatinine, ratio not elevated, full bladder | Postrenal obstruction: bladder scan or catheter and imaging |
Severity scores that use BUN
| Score | BUN input | How it is used |
|---|---|---|
| Glasgow-Blatchford score (GBS) | BUN is weighted: higher BUN adds more points | Upper GI bleeding: ACG 2021 suggests patients with GBS 0–1 may be discharged with outpatient follow-up |
| IDSA/ATS severe CAP minor criteria (2007) | BUN ≥20 mg/dL is 1 of 9 minor criteria | 1 major or ≥3 minor criteria define severe community-acquired pneumonia (CAP) |
| CURB-65 | Urea >7 mmol/L (BUN >19 mg/dL) = 1 point | CAP severity and mortality score; see the ATS/IDSA 2019 guideline for how it is used alongside other tools |
BUN is not a kidney-function test on its own: Filtration is only part of what moves the BUN. Protein load, catabolism and volume status move it too. For the kidneys themselves, go to creatinine, eGFR and urine output.
Field & Transport Considerations
Point-of-care testing
Some handheld cartridge panels report BUN with electrolytes, glucose and creatinine from a few drops of whole blood. Use them within the device's temperature and storage limits. Trend on one platform. If a result does not fit the patient, confirm it with the lab.
Before and during interfacility transport
| Setting | Consideration |
|---|---|
| Handoff | Send the latest BUN, creatinine, K⁺ and bicarbonate with draw times and the patient's baseline; for dialysis patients, the last session and any missed sessions |
| En route | BUN changes over hours to days and rarely needs a recheck in transit; monitor ECG, mental status, breathing and urine output instead |
| Red flags | Peaked T waves or arrhythmia, worsening acidosis, pulmonary edema, new confusion, pericardial rub or bleeding: notify medical control and the receiving team |
| Destination | Patients with dialysis indications need a center with nephrology and renal replacement therapy (RRT) available |
Scope: You run and read POC chemistry per local protocol (Paramedic/RN/CCT).
Special Populations
| Population | What changes | Clinical implication |
|---|---|---|
| Neonates & infants | Lower intervals: 4–15 mg/dL to 2 months, 5–15 mg/dL to 2 years | Use age-specific ranges; an adult-normal 18 mg/dL is above range for an infant |
| Children & adolescents | Adult interval from 2 years (8–23 mg/dL) | Interpret as in adults, with creatinine and hydration |
| Pregnancy | Falls through pregnancy (3–11 mg/dL in the third trimester) | A BUN near the adult upper limit is above the pregnancy range; look for volume depletion or kidney injury |
| Older adults | Same interval; less muscle mass lowers creatinine | BUN:Cr runs higher and a normal creatinine can hide a reduced GFR; use eGFR and the trend |
| CKD / dialysis | Chronic elevation that falls after each dialysis session; low-protein diets in stage 4–5 lower BUN and BUN:Cr | Compare with the patient's own baseline and the timing of the last session; a low ratio in advanced CKD can hide prerenal AKI |
| Liver disease | Reduced urea synthesis | BUN understates kidney dysfunction; rely on creatinine and urine output |
Interfering Factors & Pitfalls
Collection & handling
| Factor | Effect | Why | Avoid / fix |
|---|---|---|---|
| Drawn above a running IV | ↓ | Dilution with infusate | Draw from the other arm or per policy; repeat if implausible |
Medications & supplements
| Factor | Effect | Why | Avoid / fix |
|---|---|---|---|
| Glucocorticoids | ↑ | Protein catabolism | Expect a higher BUN:Cr; judge kidneys by creatinine |
| Tetracyclines | ↑ | More urea production | Read with creatinine |
| Parenteral nutrition, high-protein feeds | ↑ | Protein load | Note feeding changes when trending |
| Creatine supplements | BUN:Cr ↓ | Creatinine rises independent of GFR | Ask about supplements |
Physiologic / biologic
| Factor | Effect | Why | Avoid / fix |
|---|---|---|---|
| Upper GI bleeding | ↑ | Digested blood protein becomes urea | Check hemoglobin and stool |
| Trauma, burns, surgery, prolonged fasting | ↑ | Catabolism | Trend with creatinine |
| Rhabdomyolysis | BUN:Cr ↓ | Creatinine released from muscle | Check creatine kinase (CK) |
| Liver failure, malnutrition, pregnancy | ↓ | Less synthesis or higher GFR and plasma volume | Use population ranges; do not read low as healthy kidneys |
Assay & device-specific
| Factor | Effect | Why | Avoid / fix |
|---|---|---|---|
| Urea reported instead of BUN | Apparent ↑ or unit mismatch | Urea mg/dL = BUN × 2.14; SI labs report urea mmol/L (BUN × 0.357) | Check the analyte name and unit, especially on outside-hospital results |
Common misinterpretations
- Normal BUN = normal kidneys: BUN depends on protein intake and liver function. It can sit in range with a reduced GFR.
- High BUN:Cr = dehydration: GI bleeding, steroids, catabolism and low muscle mass all raise it too.
- BUN tells you when to dialyze: dialysis starts on clinical indications. A BUN number is not one of them.
- Every high BUN needs a fluid bolus: in heart failure or obstruction, fluids do not fix it, and they can harm.
Physiology & Test Basics
What it measures
BUN is the nitrogen in the urea circulating in blood. This page covers BUN alone. The BUN:creatinine ratio, creatinine and eGFR each have their own page. US labs report BUN in mg/dL. Labs on SI units report urea in mmol/L.
Physiology & regulation
- Production: the liver turns nitrogen from amino-acid breakdown into urea. Output rises with protein intake, catabolism and digested blood. It falls in liver failure.
- Excretion: the kidneys filter urea. The tubules reabsorb a variable share of it.
- Low perfusion: the renin–angiotensin–aldosterone system, sympathetic tone and vasopressin push urea reabsorption up. BUN climbs faster than creatinine as a result.
- Obstruction: tubular urea reabsorption drops, which is why the ratio is usually not raised.
- Creatinine contrast: creatinine leaves muscle at a fairly steady rate and is cleared mainly by filtration. That is what makes it the better marker of filtration.
How it is measured
- Specimen: serum or lithium-heparin plasma (green top), whichever your lab specifies. BUN sits inside the basic and comprehensive metabolic panels and the renal panel.
- Turnaround: about 45 minutes at one academic lab. Lithium-heparin plasma holds for up to 8 hr at room temperature. POC cartridges give the answer at the bedside.
Why it matters
Read beside creatinine, and BUN helps you pull low perfusion, GI bleeding and catabolism apart from intrinsic kidney injury. It also feeds risk scores for GI bleeding and pneumonia. A rising BUN with uremic symptoms is your cue for dialysis evaluation.
Clinical Pearls
New BUN rise on an anticoagulant: Before you call it dehydration, hunt for occult upper GI bleeding. Check the stool, the hemoglobin trend and orthostatic vital signs.
Pneumonia triage: A BUN of ≥20 mg/dL scores as one IDSA/ATS minor criterion. Add up the rest: respiratory rate ≥30, confusion, PaO₂/FiO₂ ≤250, multilobar infiltrates, low WBC or platelets, hypothermia, fluid-requiring hypotension. 3 or more minor criteria, or 1 major criterion, meets the IDSA/ATS 2007 definition of severe CAP.
Do not starve the kidneys: KDIGO 2012 suggests you do not restrict protein to prevent or delay dialysis in AKI. A higher BUN from adequate nutrition is expected.
Dialysis patient handoff (Transport): Write down the last session, any missed sessions and the dry weight if you know it. Potassium and volume make missed dialysis dangerous, not the urea. Keep the patient on a monitor.
Trend with intake and output (Nursing): Put BUN and creatinine on the same chart as daily weight and urine output. Anything steeper than about 10–20 mg/dL per day points to added catabolism, steroids, feeds or GI blood.
Uremic pericarditis: In kidney failure, chest pain that worsens lying flat or on inspiration, a friction rub, or signs of tamponade is an emergency. Monitor the patient, get a 12-lead ECG, bedside echo if available, and urgent nephrology for dialysis.
References
- MedlinePlus, U.S. National Library of Medicine. (2025). BUN – blood test. https://medlineplus.gov/ency/article/003474.htm
- Oregon Health & Science University Laboratory Services. (2026). CHH-Urea Nitrogen (test directory: specimen and age-specific reference ranges). https://www.ohsu.edu/lab-services/chh-urea-nitrogen
- Abbassi-Ghanavati, M., Greer, L. G., & Cunningham, F. G. (2009). Pregnancy and laboratory studies: A reference table for clinicians (landmark reference table). Obstetrics & Gynecology, 114(6), 1326–1331. https://doi.org/10.1097/AOG.0b013e3181c2bde8
- Kidney Disease: Improving Global Outcomes (KDIGO) Acute Kidney Injury Work Group. (2012). KDIGO clinical practice guideline for acute kidney injury. Kidney International Supplements, 2(1), 1–138. https://kdigo.org/wp-content/uploads/2016/10/KDIGO-2012-AKI-Guideline-English.pdf
- Malkina, A. (2026). Acute kidney injury (AKI). Merck Manual Professional Edition. https://www.merckmanuals.com/professional/genitourinary-disorders/acute-kidney-injury/acute-kidney-injury-aki
- Laine, L., Barkun, A. N., Saltzman, J. R., Martel, M., & Leontiadis, G. I. (2021). ACG clinical guideline: Upper gastrointestinal and ulcer bleeding. American Journal of Gastroenterology, 116(5), 899–917. https://doi.org/10.14309/ajg.0000000000001245
- Metlay, J. P., Waterer, G. W., Long, A. C., et al. (2019). Diagnosis and treatment of adults with community-acquired pneumonia: An official clinical practice guideline of the American Thoracic Society and Infectious Diseases Society of America. American Journal of Respiratory and Critical Care Medicine, 200(7), e45–e67. https://doi.org/10.1164/rccm.201908-1581ST
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