Lab

Blood Urea Nitrogen

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Blood Urea Nitrogen, BUN

  • Pathophysiology
  1. Increases by 10-20 mg/dl/day if Renal Function absent
  2. Serum Creatinine is a better measure of Renal Function
    1. BUN is Protein dependent
      1. High Protein diet
      2. Catabolism
    2. BUN is reabsorbed at renal tubules
      1. Prerenal Failure
        1. Dehydration
        2. Congestive Heart Failure
      2. Postrenal Failure
        1. Obstructive Uropathy
  • Lab
  1. Tube: Red Top (with serum separation) or Green Top (Heparinized, with plasma separation)
  2. Normal BUN: 8 to 20 mg/dl (2.9 mmol/L to 7.1 mmol/L)
  • Causes
  • Increased BUN
  1. Medications
    1. Aminoglycosides
    2. Diuretics
    3. Lithium
    4. Corticosteroids
  2. Increased Urea Formation
    1. Gastrointestinal Bleeding
    2. Tetracycline
    3. Full thickness Burn Injury
    4. Addison Disease
    5. High Protein Diet
  3. Decreased Renal Blood Flow (Prerenal Azotemia)
    1. Shock
    2. Congestive Heart Failure
    3. Myocardial Infarction
    4. Dehydration
    5. Renal Artery Stenosis
  4. Renal disease
    1. Glomerulonephritis
    2. Pyelonephritis
    3. Diabetic Nephropathy
  5. Urinary Tract Obstruction (Postrenal Azotemia)
    1. See Postrenal Failure Causes
  • Causes
  • Decreased BUN
  1. Liver disease
  2. Poor nutrition
  3. Overhydration
  4. Third trimester of pregnancy
  5. SIADH
  6. Anabolic Steroids
  • References
  1. Bakerman (2014) ABCs of Interpretative Lab Data, Scottsdale, Az, p. 111-2