Blood Urea Nitrogen (BUN)
Written by: Sid Ahmed Abdala Balal, RN
Medically Reviewed by: Adam Sayedi, MD
Last Updated: January 26, 2026
What is Blood Urea Nitrogen (BUN)?
Blood urea nitrogen (BUN) is a laboratory test that measures the amount of urea nitrogen in the blood. Urea is a waste product formed in the liver when dietary protein and tissue proteins are broken down. The liver converts ammonia (produced from protein breakdown) into urea through the urea cycle, and the kidneys then filter this urea from the blood and excrete it in urine.
BUN is one of the most commonly ordered tests to assess kidney function. While a small amount of urea nitrogen in the blood is normal, elevated levels may indicate impaired kidney function, dehydration, increased protein breakdown, or other medical conditions. BUN is rarely assessed alone; it is typically interpreted alongside serum creatinine and the BUN/creatinine ratio to provide a comprehensive picture of renal health.
Adults:
Males: 8-24 mg/dL
Females: 6-21 mg/dL
Children (1-17 years): 7-20 mg/dL
General range: 5-20 mg/dL
Normal ratio: 10:1 to 20:1
>20:1 suggests prerenal or postrenal causes
<10:1 may indicate acute tubular necrosis or liver disease
Venous blood sample; no fasting typically required unless part of comprehensive metabolic panel (CMP) or basic metabolic panel (BMP)
Screen for kidney disease, assess kidney function, differentiate types of azotemia, monitor dialysis effectiveness, evaluate hydration status
BUN is more variable than creatinine because it is affected by protein intake, hydration status, gastrointestinal bleeding, corticosteroids, and hepatic function. Creatinine is a more reliable indicator of kidney function because it is primarily affected by GFR and muscle mass. However, the BUN/creatinine ratio is invaluable for distinguishing prerenal azotemia (dehydration, heart failure) from intrinsic renal disease. A ratio >20:1 with normal creatinine often indicates dehydration rather than kidney damage.
Physiology of Urea Production and Excretion
Urea Synthesis in the Liver
More than 99% of urea synthesis occurs in the liver through the urea cycle. The process begins when dietary proteins are broken down into amino acids in the gastrointestinal tract. These amino acids are absorbed and transported to the liver, where they undergo deamination and transamination. The resulting ammonia (which is toxic) is converted into urea via the urea cycle.
The amount of urea produced varies based on several factors:
- Protein intake: Higher dietary protein increases urea production
- Gastrointestinal bleeding: Blood in the GI tract is digested as protein (500 mL whole blood ≈ 100 g protein)
- Catabolic states: Fever, infection, burns, trauma, and corticosteroid therapy increase protein breakdown
- Liver function: Impaired hepatic function reduces urea synthesis
- Hydration status: Dehydration concentrates BUN; overhydration dilutes it
Renal Handling of Urea
Urea is freely filtered by the glomerulus. Unlike creatinine, urea is passively reabsorbed in the renal tubules, particularly in the collecting ducts. The amount reabsorbed depends on urine flow rate:
- High urine flow (>2 mL/min): About 40% of filtered urea is reabsorbed
- Low urine flow (<2 mL/min): Up to 60% may be reabsorbed
- ADH (antidiuretic hormone): Increases collecting duct permeability to urea, enhancing reabsorption
This tubular reabsorption explains why BUN rises disproportionately to creatinine in prerenal states (dehydration, heart failure, urinary obstruction) where tubular flow is reduced and urea reabsorption is increased.
A person on a typical 70 g protein diet produces approximately 12 g of urea daily. The bulk of this urea (about 10 g/day) is excreted by the kidneys. Small amounts (<0.5 g/day) are lost through the GI tract, lungs, and skin. During exercise, a substantial fraction may be excreted in sweat.
Interpreting BUN Results
Normal BUN Values
| Age and Sex | Normal BUN Range (mg/dL) | Normal BUN Range (mmol/L) |
|---|---|---|
| Adult Males | 8-24 mg/dL | 2.9-8.6 mmol/L |
| Adult Females | 6-21 mg/dL | 2.1-7.5 mmol/L |
| Children (1-17 years) | 7-20 mg/dL | 2.5-7.1 mmol/L |
| General Reference Range | 5-20 mg/dL | 1.8-7.1 mmol/L |
Note: Reference ranges vary between laboratories. Always use your facility’s established reference intervals. BUN values are normally higher in males due to greater muscle mass and protein intake.
Elevated BUN (Azotemia)
Azotemia is defined as an elevation of BUN (typically >20-21 mg/dL) and/or creatinine. Azotemia is classified into three categories based on etiology:
Caused by decreased renal blood flow or increased urea production without intrinsic kidney damage. The kidneys are structurally normal but receive inadequate perfusion.
Characteristic findings:
- BUN/Creatinine ratio >20:1
- Urine osmolality >500 mOsm/kg (concentrated urine)
- Fractional excretion of sodium (FeNa) <1%
- Fractional excretion of urea (FeUr) <35%
- Hyaline casts on urinalysis
- Responds to fluid resuscitation or treatment of underlying cause
Common causes:
- Hypovolemia: Dehydration, hemorrhage, severe burns, excessive diuretic use, vomiting, diarrhea
- Decreased cardiac output: Congestive heart failure, cardiogenic shock, pericardial tamponade
- Renal vasoconstriction: NSAIDs, ACE inhibitors, ARBs, hepatorenal syndrome, sepsis, anesthesia
- Increased protein load: High-protein diet, gastrointestinal bleeding, corticosteroid therapy, tetracycline (except doxycycline)
- Increased catabolism: Fever, infection, trauma, burns
Caused by intrinsic kidney damage affecting the glomeruli, tubules, interstitium, or renal vasculature. The kidneys themselves are diseased.
Characteristic findings:
- BUN/Creatinine ratio 10:1 to 20:1 (relatively normal)
- Urine osmolality <350 mOsm/kg (dilute urine)
- Fractional excretion of sodium (FeNa) >2%
- Muddy brown casts (ATN), RBC casts (glomerulonephritis), WBC casts (interstitial nephritis) on urinalysis
- May not respond to fluid resuscitation
Common causes:
- Acute tubular necrosis (ATN): Ischemic (prolonged prerenal), nephrotoxic (aminoglycosides, contrast agents, rhabdomyolysis)
- Glomerulonephritis: Post-infectious, IgA nephropathy, lupus nephritis, ANCA-associated vasculitis
- Acute interstitial nephritis: Drug-induced (penicillins, NSAIDs, PPIs), autoimmune
- Chronic kidney disease: Diabetic nephropathy, hypertensive nephrosclerosis, polycystic kidney disease
- Vascular: Renal artery stenosis, renal vein thrombosis, atheroembolic disease
Caused by obstruction of urinary flow anywhere from the renal pelvis to the urethra. Bilateral obstruction or obstruction in a solitary kidney is required to cause azotemia.
Characteristic findings:
- BUN/Creatinine ratio initially >20:1 (then normalizes with prolonged obstruction as tubular damage occurs)
- Hydronephrosis on renal ultrasound or CT
- Bladder distension (if lower tract obstruction)
- Variable urine output (anuria, oliguria, or normal with partial obstruction)
- Improvement with relief of obstruction
Common causes:
- Upper tract: Kidney stones (nephrolithiasis), retroperitoneal fibrosis, tumor compression, blood clots
- Lower tract: Benign prostatic hyperplasia (BPH), prostate cancer, bladder cancer, urethral stricture, neurogenic bladder
- Bilateral ureteral obstruction: Pelvic malignancy (cervical, ovarian, colorectal cancer), retroperitoneal fibrosis
Low BUN
Low BUN levels (below 5-6 mg/dL) are less common and generally less clinically concerning, but may indicate:
- Severe liver disease: Cirrhosis, acute hepatic failure (impaired urea synthesis)
- Malnutrition or low protein diet: Inadequate protein intake reduces urea production
- Overhydration: IV fluid administration, SIADH (syndrome of inappropriate antidiuretic hormone)
- Pregnancy: Increased GFR and plasma volume expansion
- Congenital urea cycle enzyme deficiencies: Rare genetic disorders
The BUN/Creatinine ratio is calculated by dividing BUN by creatinine (both in mg/dL). Normal ratio: 10:1 to 20:1.
Ratio >20:1: Suggests prerenal azotemia (dehydration, heart failure, GI bleeding) or early postrenal obstruction. The kidneys reabsorb more urea when tubular flow is slow.
Ratio 10-20:1: Typical of intrinsic renal disease (ATN, glomerulonephritis). Both BUN and creatinine rise proportionally.
Ratio <10:1: May indicate acute tubular necrosis with reduced BUN reabsorption, liver disease (decreased urea synthesis), or rhabdomyolysis (increased creatinine production).
The BUN/Creatinine Ratio
The BUN/creatinine ratio is one of the most clinically useful tools for differentiating the cause of azotemia. It provides insight into whether the kidney dysfunction is due to factors outside the kidney (prerenal), within the kidney (renal), or downstream from the kidney (postrenal).
| BUN/Cr Ratio | Clinical Interpretation | Common Causes |
|---|---|---|
| >20:1 | Prerenal or postrenal azotemia; increased BUN reabsorption or increased BUN production | Dehydration, heart failure, GI bleeding, high-protein diet, corticosteroids, urinary obstruction (early), shock |
| 10:1 to 20:1 | Normal ratio; both BUN and creatinine rise proportionally | Intrinsic renal disease (ATN, glomerulonephritis), chronic kidney disease, normal baseline |
| <10:1 | Low BUN relative to creatinine; decreased BUN production or increased creatinine production | Severe liver disease, malnutrition, low-protein diet, SIADH, rhabdomyolysis, sickle cell anemia, dialysis (urea removed more efficiently than creatinine) |
- The BUN/Cr ratio is most useful when evaluated alongside clinical context, urinalysis, urine electrolytes (FeNa), and imaging
- A ratio >20:1 does NOT always mean prerenal azotemia; GI bleeding and corticosteroids can elevate BUN without affecting creatinine
- In chronic kidney disease, both BUN and creatinine are elevated, but the ratio often remains normal (10-20:1)
- Medications affecting creatinine secretion (cimetidine, trimethoprim) can falsely lower the ratio
- Elderly patients may have reduced muscle mass, lowering baseline creatinine and potentially elevating the ratio
A 40-year-old patient presents after running in hot weather without adequate hydration. Labs show BUN 28 mg/dL and creatinine 1.1 mg/dL. The BUN/Cr ratio is 25:1, suggesting prerenal azotemia from dehydration. After IV fluid resuscitation (1 liter normal saline), repeat labs show BUN 16 mg/dL and creatinine 0.95 mg/dL (ratio normalized to 17:1). This response confirms prerenal cause rather than intrinsic kidney disease.
Clinical Indications for BUN Testing
BUN testing is indicated in a wide variety of clinical scenarios to assess kidney function, screen for disease, and monitor treatment effectiveness.
Screening and Diagnosis
- Routine health screening: Part of basic metabolic panel (BMP) or comprehensive metabolic panel (CMP) during annual checkups
- Suspected kidney disease: Evaluate patients with symptoms of renal dysfunction (oliguria, edema, fatigue, nausea)
- Risk assessment: Screen high-risk patients with diabetes, hypertension, family history of kidney disease, or age >60 years
- Acute illness evaluation: Assess kidney function in hospitalized patients with sepsis, shock, trauma, or surgery
- Medication monitoring: Baseline and serial monitoring when prescribing nephrotoxic drugs (NSAIDs, ACE inhibitors, aminoglycosides, contrast agents)
Monitoring and Follow-Up
- Chronic kidney disease (CKD): Track progression and guide treatment adjustments
- Dialysis effectiveness: Assess adequacy of hemodialysis or peritoneal dialysis by measuring pre- and post-dialysis BUN
- Acute kidney injury (AKI): Monitor recovery or progression of kidney function
- Hydration status: Evaluate volume status in patients with heart failure, dehydration, or critical illness
- Treatment response: Monitor resolution of azotemia after interventions (fluid resuscitation, relief of obstruction, discontinuation of nephrotoxic drugs)
Symptoms Prompting BUN Testing
- Decreased urine output (oliguria or anuria)
- Dark, foamy, or bloody urine
- Swelling (edema) in legs, ankles, face, or hands
- Persistent fatigue or weakness
- Nausea, vomiting, or loss of appetite
- Shortness of breath
- Confusion or difficulty concentrating
- Chest pain or pressure (uremic pericarditis in severe cases)
- Itching (pruritus) from uremic toxins
- Muscle cramps or twitching
Nursing Care Plans for Abnormal BUN
Assessment
- Monitor vital signs: hypotension, tachycardia, orthostatic changes
- Assess skin turgor, mucous membrane moisture, capillary refill
- Monitor intake and output (I&O); calculate fluid balance
- Evaluate urine specific gravity and color (concentrated urine suggests dehydration)
- Review labs: BUN, creatinine, BUN/Cr ratio, electrolytes, hematocrit
- Assess for signs of prerenal azotemia: oliguria, weight loss, dry mucous membranes
Interventions
- Administer IV fluids as ordered (typically normal saline or lactated Ringer’s)
- Monitor fluid resuscitation response; recheck BUN/Cr after 1-2 liters
- Encourage oral fluid intake if patient able to tolerate (aim for 2-3 liters/day unless contraindicated)
- Hold nephrotoxic medications (NSAIDs, ACE inhibitors) until volume status restored
- Monitor for fluid overload during resuscitation (lung sounds, JVD, peripheral edema)
- Educate patient on adequate hydration, especially in heat or with illness
Expected Outcomes
- BUN and BUN/Cr ratio return to normal range within 24-48 hours
- Urine output >0.5 mL/kg/hr (typically >30 mL/hr for adults)
- Vital signs stabilize; orthostatic hypotension resolves
- Patient verbalizes understanding of hydration needs
Assessment
- Monitor serial BUN and creatinine levels (daily or more frequently in AKI)
- Calculate estimated GFR (eGFR) to stage kidney function
- Assess urine output hourly (normal: >0.5 mL/kg/hr; oliguria: <0.5 mL/kg/hr; anuria: <100 mL/day)
- Monitor electrolytes, especially potassium (hyperkalemia risk in kidney failure)
- Review urinalysis for casts, protein, blood
- Assess for uremic symptoms: nausea, confusion, itching, pericardial friction rub
- Identify and address reversible causes: nephrotoxic drugs, obstruction, hypotension
Interventions
- Maintain strict intake and output records; insert Foley catheter if ordered for accurate measurement
- Discontinue or adjust nephrotoxic medications per provider orders
- Ensure adequate renal perfusion: maintain MAP >65 mmHg, treat shock or hypotension
- Administer diuretics as ordered if fluid overload present (furosemide common choice)
- Implement dietary modifications: restrict potassium, phosphorus, and protein per dietitian/provider guidance
- Monitor for complications: hyperkalemia, metabolic acidosis, pulmonary edema, uremic pericarditis
- Prepare for renal replacement therapy (dialysis) if indicated: BUN >100 mg/dL, severe acidosis, hyperkalemia, uremic symptoms, volume overload unresponsive to diuretics
- Educate patient and family on kidney disease, treatment plan, and importance of follow-up
Expected Outcomes
- BUN and creatinine stabilize or trend downward
- Urine output maintained or improves to >0.5 mL/kg/hr
- Electrolytes remain within safe ranges; no life-threatening hyperkalemia
- Patient remains free from uremic complications
- If dialysis initiated, patient tolerates treatments without complications
Assessment
- Assess patient’s current understanding of kidney function and BUN test
- Identify learning barriers: language, literacy, cognitive impairment, emotional distress
- Evaluate readiness to learn and preferred learning style
- Assess support system and resources available for long-term management
Interventions
- Explain the kidney’s role in filtering waste and the meaning of elevated BUN
- Teach importance of medication adherence, especially avoiding nephrotoxic drugs (NSAIDs) without provider approval
- Educate on dietary modifications: limit protein if CKD (typically 0.6-0.8 g/kg/day), restrict potassium and phosphorus if elevated, maintain adequate hydration
- Discuss signs and symptoms requiring immediate medical attention: decreased urine output, severe swelling, shortness of breath, confusion, chest pain
- Review importance of regular follow-up lab work and nephrology appointments
- Provide written materials and resources (National Kidney Foundation, kidney disease education websites)
- Encourage questions; provide contact information for nurse or provider for concerns
Expected Outcomes
- Patient verbalizes understanding of kidney disease and BUN significance
- Patient demonstrates ability to follow dietary restrictions and medication regimen
- Patient identifies warning signs requiring medical attention
- Patient keeps scheduled follow-up appointments and lab tests
BUN Test Procedure and Preparation
Patient Preparation
- Fasting: Typically not required for BUN alone, but may be required if part of comprehensive metabolic panel (CMP) or basic metabolic panel (BMP) that includes glucose. Check facility protocol.
- Medications: Inform provider of all medications, especially those affecting kidney function (NSAIDs, ACE inhibitors, ARBs, diuretics) or urea levels (corticosteroids, tetracyclines)
- Hydration: Maintain normal hydration unless instructed otherwise; dehydration can falsely elevate BUN
- Diet: Avoid high-protein meals immediately before testing if possible, as this may temporarily elevate BUN
- Bleeding disorders: Notify lab if patient has bleeding disorder or takes anticoagulants; apply pressure longer after venipuncture
Sample Collection
- Sample type: Venous blood (serum or plasma)
- Collection tube: Red-top (serum) or green-top (heparinized plasma); avoid sodium fluoride tubes (inhibits urease enzyme)
- Sample volume: Typically 1-2 mL
- Timing: Can be drawn at any time of day; morning samples preferred if part of fasting panel
- Handling: Serum should be separated from cells promptly to prevent hemolysis; stable at room temperature for several hours
Laboratory Analysis Methods
BUN is measured using enzymatic or colorimetric methods:
- Enzymatic method (most common): Urease enzyme converts urea to ammonia and carbon dioxide. The ammonia is measured spectrophotometrically. More specific and accurate than older methods.
- Diacetyl (Fearon) reaction: Older colorimetric method; urea reacts with diacetyl to produce yellow chromogen. Less specific; can have interference from sulfonylureas and hemoglobin.
- Results timing: Typically available within 1-4 hours in most laboratories
Factors Affecting BUN Accuracy
- Medications: Corticosteroids, tetracyclines (except doxycycline), and anabolic steroids increase BUN; chloral hydrate and guanethidine may interfere with assay
- Hemolysis: Lysed red blood cells can interfere with colorimetric methods; use fresh samples
- High protein intake: Recent high-protein meal can transiently elevate BUN by 10-20%
- Dehydration: Concentrates BUN, leading to falsely elevated results not reflecting true kidney function
- GI bleeding: Blood in GI tract digested as protein, significantly elevating BUN
- Catabolic states: Fever, infection, burns, trauma increase protein breakdown and BUN
BUN in Context: Stages of Kidney Disease
BUN and creatinine are used alongside estimated glomerular filtration rate (eGFR) to classify chronic kidney disease (CKD) into stages. While eGFR is the primary staging tool, BUN provides additional context about uremia and metabolic waste accumulation.
| CKD Stage | eGFR (mL/min/1.73 m²) | Kidney Function | Typical BUN Range | Clinical Features |
|---|---|---|---|---|
| Stage 1 | ≥90 | Normal or high GFR with kidney damage | Normal (5-20 mg/dL) | Often asymptomatic; may have proteinuria, hematuria, or structural abnormalities |
| Stage 2 | 60-89 | Mild reduction in GFR | Normal to slightly elevated (10-25 mg/dL) | Usually asymptomatic; focus on slowing progression |
| Stage 3a | 45-59 | Mild to moderate reduction | Mildly elevated (20-40 mg/dL) | May develop fatigue, mild anemia, hypertension; require nephrology referral |
| Stage 3b | 30-44 | Moderate to severe reduction | Elevated (30-60 mg/dL) | Increased fatigue, edema, hypertension, anemia, bone disease; monitor closely |
| Stage 4 | 15-29 | Severe reduction | Significantly elevated (50-100+ mg/dL) | Uremic symptoms (nausea, confusion, itching); prepare for dialysis or transplant |
| Stage 5 (ESRD) | <15 | Kidney failure | Very high (>100 mg/dL) | Uremia, life-threatening complications; requires dialysis or transplant for survival |
Dialysis is typically initiated based on clinical criteria rather than BUN alone, but a BUN >100 mg/dL with uremic symptoms is a common threshold. Absolute indications for urgent dialysis include: severe hyperkalemia (K >6.5 mEq/L) unresponsive to medical therapy, pulmonary edema refractory to diuretics, severe metabolic acidosis (pH <7.1), uremic pericarditis, uremic encephalopathy, and certain intoxications (methanol, ethylene glycol, lithium, salicylates).
Frequently Asked Questions (FAQ)
References
1. Mayo Clinic. (2023). Blood urea nitrogen (BUN) test. Retrieved from https://www.mayoclinic.org/tests-procedures/blood-urea-nitrogen/about/pac-20384821
2. Cleveland Clinic. (2025). Blood Urea Nitrogen (BUN): Testing, Levels & Indication. Retrieved from https://my.clevelandclinic.org/health/diagnostics/17684-blood-urea-nitrogen-bun-test
3. Hosten, A. O. (1990). BUN and Creatinine. In H. K. Walker, W. D. Hall, & J. W. Hurst (Eds.), Clinical Methods: The History, Physical, and Laboratory Examinations (3rd ed., Chapter 193). Boston: Butterworths. Available from: https://www.ncbi.nlm.nih.gov/books/NBK305/
4. National Kidney Foundation. (2025). Blood urea nitrogen (BUN) test for kidney disease. Retrieved from https://www.kidneyfund.org/all-about-kidneys/tests/blood-urea-nitrogen-bun-test
5. Uchino, S., Bellomo, R., & Goldsmith, D. (2012). The meaning of the blood urea nitrogen/creatinine ratio in acute kidney injury. Clinical Kidney Journal, 5(2), 187-191. https://doi.org/10.1093/ckj/sfs013
6. Lerma, E. V., & Rosner, M. H. (2023). Azotemia. In StatPearls. StatPearls Publishing. Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK538145/
7. Medscape. (2024). Azotemia: Background, Pathophysiology, Etiology. Retrieved from https://emedicine.medscape.com/article/238545-overview
8. OptimalDX. (2022). Biomarkers of Kidney Function: BUN to Creatinine Ratio. Retrieved from https://www.optimaldx.com/research-blog/renal-biomarkers-bun-creatinine-ratio
Disclaimer: This content is for informational and educational purposes only and is intended for nursing professionals and students. Blood urea nitrogen (BUN) testing requires clinical correlation with creatinine, urinalysis, imaging, and patient history. Always refer to your facility’s reference ranges and consult with nephrology for complex cases involving acute kidney injury, chronic kidney disease, or unclear azotemia patterns. Not medical advice.