Blood Urea Nitrogen (BUN)
Urea is the final degradation product of protein and amino acid metabolism. In protein
catabolism the proteins are broken down to amino acids and deaminated. The ammonia formed
in this process is synthesized to urea in the liver. This is the most important catabolic pathway
for eliminating excess nitrogen in the human body. Urea is easily diffusible and exists in all body
fluids in practically the same concentration. Urea is filtered freely by the glomeruli and
reabsorbed by the proximal and distal tubules.
 BUN concentration is primarily regulated by renal tubular reabsorption, which is highly
dependent on urine flow rate.
 Increased tubular flow, due to blood volume expansion, decreases absorption and BUN
concentration.
 Decreased tubular flow, due to volume depletion or congestive heart failure, increases
absorption and BUN concentration.
 BUN concentration is also affected by protein intake (malnutrition or
hyperalimentation), endogenous protein metabolism, and liver disease.
 These variables make BUN a rather poor measure of renal function.
Creatinine is a more specific indicator of glomerular function than BUN. However, the BUN to
creatinine ratio is commonly used to determine the aetiology of acute renal failure.
 Normally, the ratio is 10 to 1. The ratio usually exceeds 20 in prerenal failure due to
decreased renal perfusion, such as occurs with hypertension, hemorrhage, or
dehydration.
 It is normal in intrinsic renal disease because BUN and creatinine rise proportionately.
 Postrenal diseases, such as urinary tract obstruction, also increase the ratio above 10
The clinical usefulness of this ratio is limited by nonrenal factors that increase BUN such as GI
bleed, parenteral nutrition, and glucocorticoid therapy.
 A GI bleed increases BUN more than creatinine because of the increased amino acid
absorption from digested blood and hypovolemia.
 Several diseases may cause a decreased BUN to creatinine ratio of less than 8 to 1.
Rhabdomyolysis results in increased production of creatinine.
 Liver disease and malnutrition can decrease the production of urea.
 Hemodialysis and peritoneal dialysis remove urea more efficiently than creatinine.
 Some medications may elevate serum creatinine by blocking tubular secretion. The
most common examples are cimetidine, trimethoprim, and pyrimethamine.
BUN correlates with uremic symptoms better than serum creatinine concentration. BUN levels
fall more rapidly than creatinine following dialysis and can be used to assess the adequacy of
dialysis.
In summary, increased BUN may be associated with renal failure, urinary tract obstruction,
dehydration and gastrointestinal hemorrhage. Decreased BUN may be associated with hepatic
failure and decreased protein intake.
Reference range
Males
0-11 months: not established
1-17 years: 7-20 mg/dL
> or =18 years: 8-24 mg/dL
Females
0-11 months: not established
1-17 years: 7-20 mg/dL
> or =18 years: 6-21 mg/dL
Specimen Type Serum or Plasma
Plain, red-top tube(s) or serum gel tube(s)
Specimen Volume: 0.5 mL of serum
Collection Instructions: Fasting. Send specimen in plastic vial.
Reject Due To
Specimens other than
Hemolysis
Lipemia
Icteric
Serum
Mild OK; Gross OK
NA
NA
Transport Temperature Refrig\Frozen OK\Ambient NO
Tietz Textbook of Clinical Chemistry. Fourth edition. Edited by CA Burtis, ER Ashwood, DE Bruns.
WB Saunders Company, Philadelphia, 2006;24:801-803