Creatinine has to do with muscle. With use, muscle cells constantly die and new ones replace them. 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 formation and its blood concentration are pretty constant.
Doctors use these BUN and serum creatinine numbers to evaluate kidney function. The BUN level can rise or fall independent of how well the kidneys are functioning, so doctors look not only at the BUN and creatinine values, but also at the ratio of BUN to 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. Lower than normal BUN values are seen in liver failure, 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 kidney function. 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 it 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.
Mitchell Hecht is a physician specializing in internal medicine. Send questions to him at: "Ask Dr. H.," Box 767787, Atlanta, Ga. 30076. Due to the large volume of mail received, personal replies are not possible.
