Blood Urea Nitrogen (BUN)

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Subject: Blood Urea Nitrogen (BUN)

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Definition

  • Protein and nucleic acid catabolism results in the formation of urea and ammonia. Urea is synthesized mainly in the liver, and >90% is excreted through the kidneys.

  • Normal range: 7–23 mg/dL.

Use

  • Most widely used screening test for the evaluation of kidney function.

  • Along with the serum creatinine, BUN levels aid in the differential diagnosis of prerenal, renal, and postrenal hyperuremia.

  • Diagnosis of renal insufficiency: filtered freely in the glomerulus; ≤50% is reabsorbed.

  • Assessment of glomerular function: A BUN of 10–20 mg/dL almost always indicates normal glomerular function.

  • In chronic renal disease, BUN correlates better with symptoms of uremia than does serum creatinine.

  • Provides evidence of hemorrhage into the upper GI tract.

  • Assessment of patients requiring nutritional support for excess catabolism, for example, burns, cancer.

Interpretation

Increased In

  • Impaired kidney function: A BUN of 50–150 mg/dL implies serious impairment of renal function. A markedly increased BUN (150–250 mg/dL) is virtually conclusive evidence of severely impaired glomerular function.

  • Prerenal azotemia—any cause of reduced renal blood flow:

    • CHF

    • Salt and water depletion (vomiting, diarrhea, diuresis, sweating)

    • Shock

  • Postrenal azotemia—any obstruction of urinary tract (increased BUN-to-creatinine ratio).

  • Increased protein catabolism (serum creatinine remains normal):

    • Hemorrhage into the GI tract

    • AMI

    • Stress

Decreased In

  • Diuresis (e.g., with overhydration, often associated with low protein catabolism).

  • Severe liver damage (e.g., drugs, poisoning, hepatitis). A low BUN of 6–8 mg/dL is frequently associated with states of overhydration or liver disease.

    • Increased utilization of protein for synthesis (e.g., late pregnancy, infancy, acromegaly, malnutrition, anabolic hormones)

    • Diet (e.g., low-protein and high-carbohydrate, IV feedings only, impaired absorption [celiac disease], malnutrition)

    • Nephrotic syndrome (some patients)

    • SIADH

    • Inherited hyperammonemias (urea is virtually absent in blood)

Limitations

  • Urea levels increase with age and protein content of the diet.

  • Corticosteroids, tetracyclines, and drugs causing nephrotoxicity frequently increase BUN.

  • The presence of ammonium ions in anticoagulants may produce falsely elevated results.

 
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