Q. What do the “BUN” and “creatinine” tests mean? Doesn’t creatinine have something to do with muscle?
—M.F., Poughkeepsie, N.Y.
A. BUN stands for “Blood Urea Nitrogen”, and represents a breakdown product of protein digestion. By the way, it’s pronounced “B-U-N”, not like “bun” you had with your last hamburger. Speaking of that hamburger, the protein in it will be digested into amino acids. These amino acids contain nitrogen, which is split off to form ammonia waste, while the rest of the amino acid is used to provide calories(fuel) for your body.
The liver helps in this protein breakdown, eventually combining the ammonia waste to form the main waste product of protein breakdown: urea. This urea is released by the liver into the blood stream as you guessed it blood urea nitrogen, or BUN. It’s then filtered and removed by the kidneys, ending up in our urine. Creatinine does indeed have to do with muscle. With use, muscle cells constantly die and are replaced with new ones. Under normal situations, muscle cells die at a pretty constant and predictable rate. When muscle cells break down, creatinine is the end waste product. Under normal conditions, creatinine’s formation and blood concentration are actually pretty constant.
Believe it or not, doctors use these BUN and serum creatinine numbers to evaluate kidney function. That’s right- two blood tests can be used to figure out kidney function! Remember, what happens to these waste products that we produce every second of every day? They’re excreted into the urine by your kidneys and urinated away. The measure of how well kid neys are working is the GFR, or “Glomerular Filtration Rate”. The two kidneys contain about 2.4 million filtration units known as “nephrons”, and each nephron contains a micro-filtration screen known as a “glomerulus.” The GFR is the best estimate of the how well the kidneys are doing their job of filtering waste and forming urine. To actually calculate this GFR is very difficult, but we can instead use the BUN and the serum creatinine levels to estimate kidney function, Kidney Diet Function!
A decrease in the GFR is associated with rises in both the BUN and creatinine levels, since if the kidney filtration rate slows down because of age or injury, less waste will be removed from the blood. However, whereas a rise in the serum creatinine always means the GFR has slowed, the BUN level can rise or fall independent of how well the kidneys are functioning. That’s why doctors look not only at the BUN and creatinine values, but also the BUN : creatinine ratio (BUN divided by creatinine).
BUN elevation out of proportion to the creatinine level might indicate a dehydrated state; gastrointestinal bleeding; heart attack; urinary tract obstruction from prostate tumor or kidney stone; shock; excessive protein intake; excessive protein breakdown from starvation; or congestive heart failure. The clinical picture usually helps doctors explain the BUN elevation. Lower than normal BUN values are seen in liver failure(a diseased liver results in decreased protein production); malnutrition; low protein diet; and over hydration.
Creatinine is produced at a fairly constant rate, so a sudden rise in its level indicates a large decline in the GFR. The normal serum creatinine level can vary depending upon how muscular a person is. A creatinine level of 1.4 in a bodybuilder may be normal; but 1.4 may represent markedly decreased kidney function in a petite elderly woman who might be expected to have a creatinine level of 1.0 or less.
Because kidneys try their best to get rid of waste when kidney function declines over time, a lot of kidney function can be lost before you’ll see a rise in the creatinine and BUN levels. That also means that when the serum creatinine and BUN rise in unison, there’s been a pretty significant decline in kidney function. Look at your labwork after reading this. Hopefully, it’ll make more sense to you. The business of making urine is a lot more complicated than most might think.