Urea

Overview

Urea measures serum concentration of this major nitrogenous waste product formed in the liver via the urea cycle from ammonia detoxification. It reflects hepatic synthetic function and renal clearance as the primary urinary nitrogen excretant. Elevated levels (azotemia) indicate renal impairment, dehydration, gastrointestinal bleeding, or high protein catabolism, while low levels suggest liver failure or malnutrition; clinically useful for assessing kidney function, monitoring acute kidney injury, and differentiating prerenal from intrinsic renal azotemia when interpreted with creatinine ratio.

Clinical Use Cases

  • Diagnosing acute and chronic kidney disease.
  • Monitoring dialysis adequacy and renal replacement therapy.
  • Evaluating prerenal azotemia from hypovolemia or heart failure.
  • Assessing gastrointestinal bleeding (BUN elevation).
  • Monitoring high catabolic states (burns, trauma, steroids).

Specimen Types

  • Serum.
  • Plasma (heparin).
  • Whole blood (point-of-care).

Measurement Methods

  • Enzymatic urease method (kinetic colorimetric).
  • Conductivity method (dialysis machines).
  • Jaffé reaction (older, less specific).

Test Preparation and Influencing Factors

  • No fasting required.
  • High protein diet, tetracycline, corticosteroids elevate levels.
  • Liver failure, malnutrition, pregnancy lower levels.
  • Upper GI bleeding increases urea via blood protein absorption.
  • Volume status affects concentration (dehydration elevates).

Synonyms

  • Blood urea nitrogen (BUN).
  • Urea nitrogen.

Further Reading