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BUN (Blood Urea Nitrogen)

Urea is a waste product, formed in the liver and excreted in the urine at a relatively constant rate.

BUN (blood urea nitrogen) is a measurement of the nitrogen part of urea, and is therefore an indirect measurement of the urea in the bloodstream.

Elevations caused by:

  • Kidney malfunction, as is seen in glomerulonephritis, pyelonephritis, and shock
  • Increased metabolism of proteins, as is seen with GI bleeding, a high protein meal, heart attack, diabetes, or infection
  • Dehydration and hemoconcentration
  • Crush injuries

Decreases caused by:

  • Liver failure
  • Normal pregnancy
  • Excessive hydration


  1. BUN is a fast way to evaluate kidney function, but has some limitations. 
  2. Small changes in BUN may occur from medications, dietary changes, and hydration status, and have little significance.
  3. Big changes in BUN occur only in fairly extreme circumstances.
  4. A more sensitive (but more complex) test of kidney function is the creatinine clearance.
  5. With an elevated BUN, one way to distinguish between renal disease and increased production of urea (such as is seen in GI bleeding) is the BUN:creatinine ratio. It increases with increased metabolism, and decreases with renal disease.

Normal Values*

  mg/dL  mmol/L
Adults 8–23 1.78.0


515 1.55.0
Pregnancy 512 1.54.5

*These are general values taken from a variety of sources. The actual normal values may vary from lab to lab and from one type of testing protocol to another.

Source: Operational Medicine 2001,  Health Care in Military Settings, NAVMED P-5139, May 1, 2001, Bureau of Medicine and Surgery, Department of the Navy, 2300 E Street NW, Washington DC, 20372-5300, USA