Creatinine and BUN tests are performed to measure renal function, filtration and over all organ health. These 2 tests, along with the creatinine clearance test, are very closely related and will help the healthcare professional understand the functional status of multiple organ systems.
They are easy and inexpensive tests to perform and frequently part of a normal blood screening.
What is BUN?
BUN stands for Blood Urea Nitrogen. It is an indirect and rough measurement of renal (kidney) and liver function, measuring the amount of urea nitrogen in the blood. (Urea is formed in the liver as the end product of protein metabolism and digestion.)
During ingestion, protein is broken down into amino acids. In the liver these amino acids are catabolized and ammonia is formed. All the ammonia molecules combine to form urea, which is then deposited in the blood and transported to the kidneys for excretion.
BUN is directly related to the metabolic function of the liver and excretory function of the kidney. Patients who have elevated BUN levels are said to have azotemia, or be azotemic.
What is Creatinine?
Creatinine tests diagnose impaired renal function and measures the amount of creatine phosphate in the blood. Creatine phosphate is used as part of skeletal muscle contraction; its daily production depends on muscle mass. Once creatine is used by the muscles it becomes creatinine and is excreted by the kidneys as BUN.
With normal kidney function, serum creatine levels should remain constant. Only in renal disorders will creatinine be abnormally elevated.
Normal levels of BUN are between 10 to 20 mg/dL for an adult and 5 to 18 mg/dL for a child. Normal creatinine levels are between .5 to 1.1 mg/dL for an adult female and .6 to 1.2 mg/dL for an adult male. Creatinine levels are lower in young children and the elderly as a result of reduced muscle mass.
Another test that often accompanies creatinine and BUN tests, is creatinine clearance. This test measures the number of milliliters of filtrate made by the kidneys per minute, called GFR. Urine and serum creatinine levels are assessed and the clearance rate is calculated.
Creatinine clearance can help clarify the extent of kidney damage and if one or both kidneys are diseased. As with creatinine, creatinine clearance is slightly lower in children and the elderly. The normal adult male can expect a range between 107 to 139 mL/min and the adult female between 87 to 107 mL/min.
As mentioned, increased creatinine levels can indicate a disease affecting renal function. It can also indicate acromegaly, gigantism or any other disease that would be associated with increased muscle mass, causing normal creatinine levels to be elevated. Decreased levels normally indicate muscle atrophy.
Increased BUN levels can indicate many more pathologies, such as shock, dehydration, congestive heart failure or even starvation. Decreased levels of BUN might be a sign of liver failure or pregnancy.