Urea Nitrogen (BUN)

CPT Code: 



Serum or Plasma 


1 mL 

Minimum Volume: 



Gel-barrier tube, green-top (lithium heparin) tube, or lavender-top (EDTA) tube; do not use ammonia heparin tube.


Spectrophotometry (SP)


Separate serum or plasma from cells within 45 minutes of collection.  Complete clot formation should take place before centrifugation.

Reference Range:

7-18 mg/dL


Temperatures  Period 
Room Temperature  3-5 Days 
Refrigerated (4⁰C)  7 Days 
Frozen (-20 ⁰C) Indefinitely 


BUN is a waste product produced by the liver during protein breakdown and excreted by the kidneys. BUN is most commonly measured in the diagnosis and treatment of certain renal and metabolic diseases. 

Clinical Significance: 

High Urea Nitrogen occurs due to a high-protein diet or excessive destruction of cellular proteins as occurs in massive infection and fevers, reduced renal perfusion resulting from dehydration or heart failure, nearly all types of kidney disease, and mechanical obstruction to urine excretion such as is caused by stones, tumors, infection, or stricture

Low Urea Nitrogen occurs in normal pregnancy, decreased protein intake, with intravenous fluids, with some antibiotics, overhydration and in some but not all instances of liver disease.

Causes for Rejection: 

Improper labeling 

Interfering Substances: 

This method reacts quantitatively with ammonium ions.

Triglyceride of 1000 mg/dL, the magnitude of the interference could not be determined.