Blood Urea Nitrogen (BUN)

Overview

Blood urea nitrogen (BUN) measures the amount of urea nitrogen, a waste product from protein metabolism, in the blood. It reflects kidney function in excreting urea, hydration status, and protein catabolism rates. Elevated levels may indicate kidney impairment, dehydration, gastrointestinal bleeding, or high protein intake, while low levels can signal malnutrition, liver disease, or overhydration. BUN is clinically useful for assessing renal function, especially alongside creatinine in the BUN/creatinine ratio.kidney+1

Clinical Use Cases

  • Evaluating acute or chronic kidney disease.
  • Monitoring dehydration or fluid status.
  • Assessing gastrointestinal bleeding (elevated BUN).
  • Differentiating prerenal from intrinsic renal azotemia.

Specimen Types

  • Serum.
  • Plasma (lithium heparin, EDTA in some assays).
  • Whole blood (point-of-care).

Measurement Methods

  • Enzymatic conductivity rate method.
  • Urease-indophenol method (colorimetric).
  • Jaffé kinetic method (less common).

Test Preparation and Influencing Factors

  • No fasting required.
  • High-protein diet, tetracycline, or corticosteroids elevate levels.
  • Liver disease, low-protein diet, or pregnancy lower levels.
  • Dehydration increases; overhydration decreases BUN.

Synonyms

  • Urea nitrogen.
  • BUN.
  • Serum urea nitrogen (SUN).

Further Reading

  • BUN (Blood Urea Nitrogen): MedlinePlus Medical Test; MedlinePlus;