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What is blood urine nitrogen (BUN)?
Blood urea nitrogen is a measure of the amount of nitrogen in the blood that
comes from the waste product urea and a measurement of renal function. Urea is a
waster product of protein metabolism by the liver, and removed from the blood by
the kidneys.
Normal Range:
Generally, the normal range is 2.9-7.1mmol/L (8.0-20mg/L);
Normal range for people elder than 60 years old: 2.8-7.8mmol/L
(7.8-21.8mg/L).
The adult BUN level grows with age. Men may have slightly higher values than
women. The BUN level decreases by 25 percent during pregnancy. The newborn
infants have a lower level than adults.
Clinical significance of BUN:
Increased BUN:
(1) Renal increase of BUN is caused by renal dysfunction due to Acute or
Chronic Nephritis, Poisoning Nephritis, serious Pyelonephritis, Nephrophthisis,
renal vascular sclerosis, congenital Polycystic Kidney or kidney neoplasms
etc.
High BUN level has a special diagnostic value to Uremia, because its
increased degree is consistent with the severity of disease. For example: In
azotemia period of Renal Failure, BUN is higher than 9mmol/L. In Uremia period,
BUN level can be higher than 20mmol/L. Therefore, BUN teat is helpful to
diagnosis.
(2) Pre-renal increase of BUN is usually caused by congestive heart failure,
severe burn, shock, hemorrhoea in digestive tract, serious infection, diabetic
acidosis, decline of adrenal cortex function, hepatorenal syndrome and so
on.
(3) Post-renal increase of BUN is commonly caused when glomerular Filtration
Rate (GFR) declines due to obstruction of urinary tract increases the pressure
in renal tissue, such as obstruction of urinary tract caused by prostatic
hyperplasia or oncothlipsis, calculus in bilateral ureters.
Vasoactive substances such as prostaglandins and angiotensin are released
during obstruction of the urinary system. These substances cause constriction of
the glomerular arterioles, reducing the blood flow and decreasing the glomerular
filtration rate (GFR). In turn, the decreased GFR prevents clearance of BUN,
causing increased concentrations of these substances in the blood.
Decreased BUN:
Low BUN level has little significance for diagnosis. It occasionally can be
seen with acute hepatatrophy, toxic hepatitis, lipid nephropathy.
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