Why BUN-creatinine is measured
BUN-creatinine ratio and acute renal failure
How to prepare for a BUN-creatinine ratio test
Risks associated with testing the BUN-creatinine ratio
Blood Urea Nitrogen (BUN) and creatinine are both waste products of normal metabolism in the human body. BUN represents the amount of nitrogen produced from the metabolism of proteins. Creatinine is a normal waste product of muscle. The ratio of BUN to creatinine is a relationship between these two end products found in blood, and paints a clinical picture for physicians describing kidney functionality.
The BUN-creatinine ratio provides specific clinical information about the kidney that can be used for multiple purposes. The BUN-creatinine ratio is obtained to assess normal kidney function, help identify possible kidney diseases, to monitor the progression of kidney disease, or to monitor the effectiveness of medications in treating kidney disease.
BUN-creatinine ratio is obtained whenever medically appropriate regardless of age, gender, or race. It is commonly measured in patients before a surgical procedure to assess general function, after some types of surgical procedures, and in patients with kidney disease or failure. The BUN-creatinine ratio is also obtained to assess the degree and effectiveness of kidney filtration prior to some radiology studies.
The BUN-creatinine ratio is a value measured in the blood to help assess the health of the kidney.
As a normal part of protein metabolism in the liver, protein is broken down into a compound called urea. Urea can be measured by nitrogen in the blood. Once in the blood, urea nitrogen is carried to the kidneys to be filtered out of the blood into the urine. Some of the urea nitrogen is reabsorbed back into the body for further use, but most is left in the urine. Hence, products of protein breakdown are excreted from the body. Females have lower normal BUN values than males. Women in the second or third trimester of pregnancy may present with a lower BUN value as normal. Normal BUN values in the elderly may be elevated.
Abnormal BUN Values
If the BUN value not within the normal range, it is a sign that either there is an excess of protein breakdown products in the blood, or some part of the system of BUN filtration is not functioning normally. A BUN value higher or lower than normal demonstrates a potential problem in urea removal from the blood.
The BUN filtration system relies on both kidney filtration function and on a sufficient amount of blood traveling to the kidney to be filtered. An abnormal BUN value may indicate a breakdown somewhere in this system. The kidney may not be filtering properly due to disease or injury, or blood flow may be decreased to the kidney and so not available for filtration. Either of these scenarios may cause an increased BUN value. Kidney damage can be caused by diabetes, high blood pressure, and pathologies that block the urinary tract such as kidney stones. Decreased blood flow to the kidneys may be caused by multiple disease states or physiological disorders, including congestive heart failure, dehydration, shock, or gastrointestinal bleeding.
Diets that involve consuming large amounts of protein, such as the Atkins diet, may cause an excess of protein breakdown products to be present in the blood. The more protein is ingested, the greater the amount of urea and nitrogen will be present in the blood for the kidneys to filter. Even with a healthy pair of kidneys, an excess amount of protein to be filtered can stress the kidney filtration system and result in higher than normal BUN levels. Lower than normal BUN values may occur due to a state of malnutrition where not enough protein is ingested, over hydration where the amount of BUN is diluted, or liver damage where protein breakdown is defective.
Creatinine is a waste product formed when the amino acid creatine in muscle tissue is metabolized. Creatinine is released into the blood, where it is carried to the kidneys for filtration into the urine. When creatinine levels are measured in the blood it is known as serum creatinine (used to help calculate the BUN-creatinine ratio). When creatinine levels are measured in the urine it is known as urine creatinine. Once creatinine is filtered by the kidney, it is not reabsorbed into the body. All the creatinine that is filtered is excreted in the urine. This is an important trait of creatinine, and makes it useful as a monitor of kidney filtration capability. Since creatinine origates from muscle breakdown, females may have lower normal serum creatinine values than males due to lower muscle mass. Pregnancy may cause low normal serum creatinine.
Abnormal Creatinine Values
Some types of kidney disease affect the ability of the kidney to filter waste products such as creatinine from the blood. The kidney diseases acute tubular necrosis, diabetic nephropathy, and glomerulonephritis may all increase serum creatinine values. Diseases that decrease the amount of blood that reaches the kidney for filtration may also increase serum creatinine values. The muscle disease rhabdomyolysis and any other disease that causes excessive breakdown of muscle tissue can cause abnormally high serum creatinine levels. Pathologies that block the urinary tract such as kidney stones will increase serum creatinine. Lower than normal serum creatinine values may be caused by decreased muscle mass due to age or diseases such as muscular dystrophy. Very low protein diets may also cause decreased serum creatinine levels.
The BUN-creatinine ratio is determined by measuring the concentrations of BUN and creatinine in the blood. A change in either component will influence the value of the ratio. A normal BUN-creatinine ratio is based on the normal values for BUN and serum creatinine. A normal BUN value is 10–20 mg/dl. A normal serum creatinine value is 0.5–1.2 mg/dl. Hence, a normal BUN-creatinine ratio lies between 10:1 and 20:1. The normal value of the BUN-creatinine ratio is different in infants less than 12 months old, where it may be as high as 30:1 and still be normal.
Abnormal BUN-creatinine ratio values
Abnormal BUN-creatinine ratios may be caused by many different types diseases, disorders, or injury to the kidney. The ratio may be abnormally high with any pathology that increases BUN or decreases creatinine. The ratio may be abnormally low with any pathology that decreases BUN or increases creatinine.
Acute Renal Failure (ARF), or kidney failure, may be caused by kidney disease or injury. The cause of ARF may be due to a pathology that occurs outside of the kidney before the blood reaches the kidney filtration apparatus, or within the actual kidney. Depending on where the cause for kidney failure lies, ARF is categorized as being “prerenal” or “renal or intrinsic.” The BUN-creatinine ratio is a useful tool for identifying which category of ARF is present in a patient. Prerenal causes of ARF create extremely high BUN-creatinine ratios. Renal or intrinsic causes of ARF create BUN-creatinine ratios that are higher than normal but less than those created by prerenal ARF.
BUN-creatinine ratios are measured using blood samples. Having blood drawn from a vein with a syringe, usually in the arm, is necessary. Since some medications may affect the results, it is critical that the physician take into account all prescription medications, non-prescription medications, herbal, and nutritional supplements that the patient is taking before running the BUN-creatinine ratio test. Age and gender may also affect the results in predictable patterns. To prepare for the BUN-creatinine test patients should not do any strenuous exercise for 2 days (48 hours) prior; not eat meat, especially beef, or other protein for 24 hours prior; drink a normal amount of fluids.
Drugs that affect BUN
- Amphotericin B;
- High-Dose Aspirin;
- Polymixin B;
- Thiazide Diuretics;
- Triamterene; and
QUESTIONS TO ASK YOUR DOCTOR
- Why do I need the BUN-creatinine ratio measured?
- What results are anticipated from my test?
- When can I expect the results?
- Are there any risks associated with the test?
- What do I need to do to prepare for the test?
- Will any of my medications, nutritional, or herbal supplements affect the test?
Drugs that affect creatinine
- Trimethoprim; and
- Any drug toxic to kidneys.
There is very little risk associated with having blood drawn for a BUN-creatinine ratio test. Most people have no side effects or a small bruise; however, with any blood draw there is a small chance that the area around the punctured vein may develop phlebitis, the inflammation of a vein. Phlebitis may also involve a bacterial infection if the site of the blood draw was not appropriately cleaned before the needle was inserted. Phlebitis can be locally painful but usually resolves in a short period of time. Additionally, patients with disorders involving the inability of the blood to form normal blood clots should discuss their condition and their medications with the physician before the blood draw and BUN-creatinine ratio test is done.
Risks associated with the test result include a deceptively normal value for the BUN-creatinine ratio. It is important that the physician note both the value of the BUN-creatinine ratio and the values of BUN and serum creatinine individually. Kidney damage may present with abnormal values for BUN and creatinine, but a normal value for their ratio. If the individual values are not noted, this scenario may present a seemingly normal picture of health that is not accurate. For example, in patients with chronic kidney failure, the BUN-creatinine ratio may be 10:1
Acute Renal Failure (a.k.a. Acute Kidney Failure)— The rapid loss of the kidney’s ability to function due to kidney damage. Acute renal failure decreases the kidney’s ability to filter waste products from the blood for excretion in the urine.
Acute Tubular Necrosis— A kidney disease involving damage to the portion of the kidney known as the tubules that causes kidney failure.
Atkins Diet— A diet that involves eating a high amount of protein and fat with a low amounts of carbohydrates.
Blood Serum— The fluid portion of the blood.
Congestive Heart Failure— A serious condition caused by disease or damage to the heart that weakens the heart’s ability to pump a sufficient amount of blood to the body tissues.
Diabetic Nephropathy— A progressive kidney disease associated with diabetes that interferes with the kidney’s ability to function in filtering waste products from the blood.
Gastrointestinal Tract— The path in the body from the mouth, through the stomach, intestines, rectum, and the anus.
Glomerulonephritis— A disease of the kidneys mediated by the immune system that causes inflammation of the part of the kidney known as the glomerulus.
Muscular Dystrophy— A genetic muscle disease that causes progressive muscle weakness along with the breakdown and death of muscle tissue.
Phlebitis— Inflammation of a vein.
Rhabdomyolysis— A condition causing the rapid breakdown of muscle tissue that may be caused by severe injuries or toxic chemicals. It causes the release of muscle tissue breakdown products into the blood in such excess that it may lead to acute renal failure.
with a BUN level of 60mg/dl and a serum creatinine level of 2mg/dl. In conclusion, a normal looking BUN-creatinine ratio does not always mean that the kidneys are functioning normally.
Chaudhry, H. J., et al. Fundamentals of Clincal Medicine, 4th ed. Philadelphia: Lippincott Williams & Wilkins, 2004.
Gennari, F. J. Medical Management of Kidney and Electrolyte Disorders. New York: Informa Healthcare, 2001.
Le, Tao, Vikas Bhushan, and Chirag Amin. First Aid for the Wards, 2nd ed. New York: McGraw-Hill, 2002.
Maxwell, R. W. Maxwell Quick Medical Reference, 5th ed. Tulsa, OK: Maxwell Publishing Company, 2006.
“Blood Urea Nitrogen.” WebMD. August 21, 2006. http://www.webmd.com/a-to-z-guides/blood-urea-nitrogen (April 8, 2008).
“Creatinine and Creatinine Clearance.” WebMD. August 21, 2006. http://www.webmd.com/a-to-z-guides/creatinine-and-creatinine-clearance (April 8, 2008).
Maria Basile, Ph.D.
BUN test seeKidney function tests