The BUN-to-creatinine ratio is a routine lab measurement used by your doctor to assess the function of your kidneys. It is calculated by dividing the serum BUN concentration by the serum creatinine concentration. Various conditions affecting your kidneys will alter this ratio in a predictable manner before you even show symptoms. Therefore, it is a useful ratio when monitoring subtle changes in your kidney health.
Your kidneys are capable of sustaining significant damage before showing any clinical symptoms, such as changes in urine volume or color. This is because kidneys are composed of an abundance of nephrons, which help produce urine. The number of nephrons you are born with is much greater than the number you need to survive. This creates a large buffer for kidney damage; therefore, while you may not be exhibiting any obvious symptoms, your kidneys may be under attack from an unknown disease process.
BUN stands for Blood Urea Nitrogen. This molecule is a product of protein breakdown and is typically present in your blood at a stable concentration. Generally, the BUN concentration is related to your total muscle mass, as muscles are constantly undergoing breakdown and rebuilding processes, releasing nitrogen-rich protein into your blood. BUN is a waste product, but its excretion is not actively controlled. Its ability to permeate membranes allows us to use it as a measurement of the rate of urine flowing through nephrons.
Creatinine is also released into the blood by muscle, but its properties allow the kidney to handle it in a different way. Nephrons are capable of actively reabsorbing and secreting creatinine. By measuring the amount of cretinine in your blood, your doctor can assess how your kidneys are handling this unique molecule.
The ratio of BUN to creatinine in blood changes in a predictable way when different diseases affect the kidney. According to Mayo Clinic, variation in the BUN-to-creatinine ratio can often indicate to a physician not only that a kidney problem exists, but also approximately where the problem is located.
A normal BUN:creatinine ratio is 15:1, according to "Renal Pathophysiology." When the ratio approaches 20:1, it suggests a problem with bloodflow to the kidney; this may be caused by conditions such as heart failure or shock. Alternatively, a BUN:creatinine ratio of 10:1 suggests an intrinsic renal disease, such as glomerulonephritis or acute tubular necrosis.