Blood Urea Nitrogen is the outcome which is produced along with the metabolism of protein in human body. NH3 and CO2 in liver can be composed into urea (in general, there is 0.3g urea in every gram of protein after the metabolism). Kidney is the main organ that can excrete urea. Urea is filtrated in the glomeruli and is reabsorbed in the renal tubules.
The reference range of BUN in normal adult without food is 3.2-7.1mmol/L (9-20mg/dL in another unit, and this range varies in different regions due to different approaches of detection). In normal condition, the ratio of BUN in serum creatinine is about 10.
The same as serum creatinine, the value of BUN can be an indicator of kidney functions. Usually, when kidney is only damaged slightly, the value of BUN is still in the normal range. Only when the glomerular filtration rate (GFR) is lower than 50%, will the BUN increase obviously. Generally speaking, various kinds of pathological changes in renal parenchyma will make the BUN rises, and there are many influencing factors that can affect the value of BUN. For instance, high-protein diet, water shortage, ischemia of kidney and some types of acute glomerulonephritis will elevate the value of BUN to be as high as 20-30. While, when the intake of protein is too low, or there is something wrong with liver, the value may decrease.
We say above that BUN is related to kidney function, and then, how does it indicate the function?
When the value of BUN increases, we can probably know the organic changes in renal parenchyma, including primary glomerulonephritis, inerstitial nephritis, chronic renal failure caused by polycystic kidney disease, etc. Secondly, it will indicate the prerenal oliguria aroused by mass ascites, failure in heart funtion or pathological changes in liver and so on. At last it may present the excessive intake or decomposition of protein. Meanwhile, when the value of BUN decreases, we can know that there is lesion in renal tubules, because when the filtration function of glomeruli dicreases, the concentration of BUN as well as serum creatinine will stay in the kidneys. However, we can not ensure the disease condition only depend on the sole value of BUN, since diagnosis should be a comprehensive process which requires great responsibility.
