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Quick Reference
  • Normal Range: 6-24 mg/dL (varies by laboratory; some use 7-20 mg/dL)
  • Critical Values: >100 mg/dL (consider uremia/dialysis evaluation)
  • Primary Use: Assessment of kidney function and hydration status
  • Sample Type: Serum (venous blood)
  • BUN:Cr Ratio: Normal 10-20:1; ratio >20:1 suggests prerenal azotemia
  • Elevated Causes: Dehydration, kidney disease, GI bleeding, high protein diet
  • Key Point: Less specific than creatinine; value is in BUN:Cr ratio for differentiating azotemia types

Test Description

What is BUN?

Blood Urea Nitrogen (BUN) measures the amount of nitrogen in the blood that comes from urea. Here's how it works:

  • Protein metabolism: When the liver breaks down proteins, urea is produced as a waste product
  • Release: Urea is released from the liver into the bloodstream
  • Filtration: Urea travels to the kidneys where it is filtered out
  • Excretion: Filtered urea is excreted in urine

BUN and Kidney Function

BUN is one of the most commonly ordered tests to assess kidney function, though it is less specific than creatinine.

BUN levels can be affected by factors beyond kidney function:

  • Hydration status (dehydration raises BUN)
  • Protein intake (high protein diet raises BUN)
  • Liver function (liver disease lowers BUN production)
  • GI bleeding (digested blood acts as protein load)
BUN vs Creatinine: While both assess kidney function, creatinine is more specific for kidney disease because it is less affected by diet and hydration. BUN is more useful for assessing volume status and differentiating types of azotemia when combined with creatinine (BUN:Cr ratio).
Normal Reference Ranges
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Population Normal Range (mg/dL) Notes
Adults 6-24 mg/dL Most common reference; some labs use 7-20 mg/dL
Elderly May be slightly higher Due to decreased muscle mass and GFR
Children 5-18 mg/dL Lower due to smaller protein mass
Pregnancy Lower than non-pregnant Increased GFR and hemodilution
SI Units: BUN can also be reported as urea in mmol/L (multiply mg/dL by 0.357). Always check your laboratory's reference range.
Clinical Significance

Elevated BUN (Azotemia) Causes

Prerenal Azotemia (decreased renal perfusion - BUN:Cr ratio >20:1)

  • Dehydration / volume depletion
  • Congestive heart failure
  • Shock (cardiogenic, septic, hypovolemic)
  • Renal artery stenosis
  • GI bleeding (protein load from digested blood)
  • High protein diet or catabolic states

Intrinsic Renal Azotemia (kidney damage - BUN:Cr ratio 10-20:1)

  • Acute kidney injury (AKI) - acute tubular necrosis
  • Chronic kidney disease (CKD)
  • Glomerulonephritis
  • Interstitial nephritis
  • Nephrotoxic drugs (NSAIDs, aminoglycosides, contrast)

Postrenal Azotemia (urinary obstruction - BUN:Cr ratio 10-20:1)

  • Kidney stones (bilateral or single kidney)
  • Prostatic hypertrophy or malignancy
  • Bladder outlet obstruction
  • Retroperitoneal fibrosis or tumors

Decreased BUN Causes

  • Liver failure (unable to produce urea)
  • Malnutrition or low protein diet
  • Overhydration / SIADH
  • Pregnancy (increased GFR, hemodilution)
Interpretation Guidelines

Using BUN:Creatinine Ratio to Differentiate Azotemia

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BUN:Cr Ratio Interpretation Common Causes
>20:1 Prerenal azotemia Dehydration, CHF, GI bleeding
10-20:1 Intrinsic or postrenal azotemia AKI, CKD, obstruction
<10:1 Low protein states or liver disease Malnutrition, cirrhosis, overhydration
Clinical Pearl: A BUN:Cr ratio >20:1 with elevated BUN suggests volume depletion or decreased renal perfusion (prerenal). This often responds to IV fluids. A ratio 10-20:1 with both elevated suggests intrinsic kidney disease.

Severity Classification

  • Mild Elevation: 25-40 mg/dL - May indicate early kidney dysfunction or mild dehydration
  • Moderate Elevation: 40-100 mg/dL - Significant kidney impairment or volume depletion
  • Severe Elevation: >100 mg/dL - Advanced kidney failure; consider dialysis evaluation if symptomatic uremia
Interfering Factors
  • Diet: High protein diet increases BUN; low protein diet decreases it
  • Hydration: Dehydration falsely elevates BUN; overhydration lowers it
  • Medications: Corticosteroids, tetracyclines increase BUN; anabolic steroids decrease it
  • GI Bleeding: Digested blood acts as a protein load, elevating BUN disproportionately to creatinine
  • Muscle Mass: Low muscle mass (elderly, cachectic) may result in lower baseline BUN
  • Liver Disease: Severe liver dysfunction reduces urea production
Clinical Pearls
BUN:Cr Ratio Is Key: Never interpret BUN in isolation. Always calculate the BUN:creatinine ratio to differentiate prerenal (>20:1) from intrinsic/postrenal (10-20:1) azotemia. This helps guide treatment (fluids vs other interventions).
GI Bleeding Raises BUN: Upper GI bleeding causes disproportionate BUN elevation compared to creatinine because digested blood is absorbed as protein. Look for BUN:Cr >30:1 in significant GI bleeds.
Creatinine Is More Specific: BUN is affected by many non-renal factors (diet, hydration, GI bleed, catabolism). Use creatinine and eGFR for more accurate kidney function assessment. BUN's value is in the BUN:Cr ratio.
Prerenal Responds to Fluids: If BUN:Cr >20:1 with elevated BUN suggests prerenal azotemia, try a fluid bolus. If BUN improves and ratio normalizes, prerenal etiology is confirmed. If not, consider intrinsic kidney disease.
Elderly Baseline Higher: Elderly patients often have slightly elevated baseline BUN due to decreased GFR and muscle mass. Establish baseline values in stable patients to detect acute changes.
References
  1. Kratz, A., Ferraro, M., Sluss, P. M., & Lewandrowski, K. B. (2004). Laboratory reference values. New England Journal of Medicine, 351, 1548-1564.
  2. Lee, M. (Ed.). (2009). Basic skills in interpreting laboratory data. Ashp.
  3. Farinde, A. (2021). Lab values, normal adult: Laboratory reference ranges in healthy adults. Medscape.
  4. Nickson, C. (n.d.). Critical Care Compendium. Life in the Fast Lane • LITFL.
  5. Farkas, Josh MD. (2015). Table of Contents - EMCrit Project. EMCrit Project.
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