Another substance, creatine (say "KREE-uh-teen"), is formed when food is changed into energy through a process called metabolism. Creatine is broken down into creatinine. Your kidneys take creatinine out of your blood and pass it out of your body in urine.
If your kidneys are damaged and can't work as they should, the amount of creatinine in your urine goes down while its level in your blood goes up.
Three types of tests can be done.
Blood creatinine level
The blood creatinine level shows how well your kidneys are working. A high level may mean your kidneys are not working as they should. The amount of creatinine in the blood depends partly on the amount of muscle tissue you have. Men generally have higher creatinine levels than women.
A creatinine clearance test measures how well creatinine is removed from your blood by your kidneys. This test gives better information than a blood creatinine test on how well your kidneys are working. The test is done on both a blood sample and on a sample of urine collected over 24 hours.
Blood urea nitrogen-to-creatinine ratio (BUN:creatinine)
The BUN test measures the amount of urea in your blood. Urea is a waste product made when protein is broken down in your body. Urea is made in the liver and passed out of your body in the urine.
The levels of blood creatinine and blood urea nitrogen (BUN) can be used to find the BUN-to-creatinine ratio. This ratio can help your doctor check for problems, such as dehydration, that may cause abnormal BUN and creatinine levels.
Why It Is Done
These tests are done:
- To see if your kidneys are working normally.
- To find out if your kidney disease is changing.
- To see how well the kidneys work in people who take medicines that can cause kidney damage.
- To check for severe dehydration. Dehydration generally causes BUN levels to rise more than creatinine levels. This causes a high BUN-to-creatinine ratio. Kidney disease or blocked urine flow from your kidney causes both BUN and creatinine levels to rise.
How To Prepare
Do not do any strenuous exercise for 2 days (48 hours) before having creatinine tests.
Do not eat more than 8 oz (227 g) of meat, especially beef, or other protein for 24 hours before the blood creatinine test and during the creatinine clearance urine test.
Be sure to drink enough fluids during the 24-hour urine collection, but do not drink coffee or tea. These are diuretics that cause your body to pass more urine.
How It Is Done
Collecting the blood sample
The health professional drawing blood will:
- Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
- Clean the needle site with alcohol.
- Put the needle into the vein. More than one needle stick may be needed.
- Attach a tube to the needle to fill it with blood.
- Remove the band from your arm when enough blood is collected.
- Put a gauze pad or cotton ball over the needle site as the needle is removed.
- Put pressure on the site and then put on a bandage.
Collecting the 24-hour urine sample
- You start collecting your urine in the morning. When you first get up, empty your bladder but do not save this urine. Write down the time that you urinated. This marks the beginning of your 24-hour collection period.
- For the next 24 hours, collect all your urine. Your doctor or lab will usually provide you with a large container that holds about 1 gal (4 L). The container has a small amount of preservative in it. Urinate into a small, clean container and then pour the urine into the large container. Do not touch the inside of the container with your fingers.
- Keep the large container in the refrigerator for the 24 hours.
- Empty your bladder for the final time at or just before the end of the 24-hour period. Add this urine to the large container, and record the time.
- Do not get toilet paper, pubic hair, stool (feces), menstrual blood, or other foreign matter in the urine sample.
How It Feels
The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.
This test does not cause any pain.
Risks of a blood test
There is very little chance of a problem from having a blood sample taken from a vein.
- You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
- In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
- Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.
Collecting a 24-hour urine sample does not cause problems.
Creatinine and creatinine clearance tests measure creatinine levels in your blood and urine. These tests give information about how well your kidneys are working. The creatinine clearance value is found from the amounts of creatinine in the urine and blood and from the amount of urine you pass in 24 hours. This value is the amount of blood cleared of creatinine per minute, based on your body size.
These numbers are just a guide. The range for "normal" varies from lab to lab. Your lab may have a different range. Your lab report should show what range your lab uses for "normal." Also, your doctor will evaluate your results based on your health and other factors. So a number that is outside the normal range here may still be normal for you.
Women: 0.5-1.1 mg/dL or 44-97 mcmol/L
Teen: 0.5-1.0 mg/dL
Child: 0.3-0.7 mg/dL
Men (younger than age 40): 107-139 milliliters per minute (mL/min) or 1.8-2.3 milliliters per second (mL/sec)
Women (younger than age 40): 87-107 mL/min or 1.5-1.8 mL/sec
Creatinine clearance values normally go down as you get older. (Normal values go down by 6.5 mL/min for every 10 years past the age of 20.)
6-25, with 15.5 being the best value
- High creatinine blood levels can be caused by:
- Serious kidney damage or chronic kidney disease. Kidney damage can be caused by a life-threatening infection, shock, cancer, or low blood flow to the kidneys.
- Muscle injury and conditions. These include crush injuries, burns, rhabdomyolysis, muscular dystrophy, polymyositis, and strenuous exercises.
- Shock. This is low blood pressure with many causes including severe bleeding and very severe infection.
- High creatinine clearance levels can be caused by carbon monoxide poisoning, hypothyroidism, and pregnancy.
- High BUN-to-creatinine ratios occur with sudden (acute) kidney problems. This may be caused by shock or severe dehydration. A very high BUN-to-creatinine ratio may be caused by bleeding in the digestive tract .
- Low blood creatinine levels can mean lower muscle mass caused by a disease, such as muscular dystrophy, or by aging. Low levels can also mean some types of severe liver disease or a diet very low in protein. Pregnancy can also cause low blood creatinine levels.
- Low creatinine clearance levels can mean you have chronic kidney disease or serious kidney damage. Kidney damage can be from conditions such as a life-threatening infection, shock, cancer, low blood flow to the kidneys, or urinary tract blockage. Other conditions, such as heart failure and dehydration, can also cause low creatinine clearance levels.
- Low BUN-to-creatinine ratios may be linked with a diet low in protein, a severe muscle injury called rhabdomyolysis, pregnancy, cirrhosis, or syndrome of inappropriate antidiuretic hormone secretion (SIADH). SIADH sometimes occurs with lung disease, cancer, diseases of the central nervous system, or the use of certain medicines.
What Affects the Test
You may not be able to have the test, or the results may not be helpful, if:
- You take certain medicines, such as:
- You do some strenuous exercise 2 days before a creatinine clearance test.
- You eat more than 8 oz (227 g) of meat, especially beef, in the 24 hours before a blood creatinine test or during a creatinine clearance urine test.
- You don't collect your urine for exactly 24 hours.
What To Think About
- A high blood creatinine level is generally seen with a low creatinine clearance level. This is because creatinine in the blood is normally removed by the kidneys. If the kidneys are not able to remove creatinine (low creatinine clearance), levels of creatinine in the blood go up (high blood creatinine level).
- If you are pregnant, your doctor can check the amount of creatinine in amniotic fluid to see how developed your baby's kidneys are. This can be helpful if there is a chance your baby will be delivered early. A baby whose kidneys are fully developed will make more creatinine than a baby whose kidneys are still developing.
- A normal blood creatinine level does not rule out kidney disease. To help see if there is kidney damage, creatinine clearance is also measured. Other tests may also be done to check for kidney disease. For more information, see the topic Blood Urea Nitrogen.
- Creatinine levels increase more slowly than blood urea nitrogen (BUN) levels. So an increase in creatinine may mean chronic kidney problems.
- A glomerular filtration rate may be done for people with chronic kidney disease. This test checks how well the kidneys are working.
- Diabetes experts recommend that blood creatinine levels be checked every year for people with diabetes. The creatinine level is used to find the glomerular filtration rate.
- The amount of creatinine in the blood depends partly on the amount of muscle tissue. This means that creatinine levels are generally higher in men than in women. Also, people who have large muscles, such as athletes, normally have above-average blood creatinine levels.
- A one-time urine sample to measure urine creatinine and sodium is sometimes done along with creatinine and sodium blood tests. This helps find the fractional excretion of sodium (FENa). This test can help your doctor see if a problem with blood flow to the kidneys is caused by dehydration or shock or by damage to the kidneys themselves.
Other Works Consulted
American Diabetes Association (2015). Standards of medical care in diabetes-2015. Diabetes Care, 38(Suppl 1): S1-S93.
Ceriotti F, et al. (2008). Reference intervals for serum creatinine concentrations: Assessment of available data for global application. Clinical Chemistry, 54(3): 559-566.
Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.
Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
Primary Medical ReviewerE. Gregory Thompson, MD - Internal Medicine
Kathleen Romito, MD - Family Medicine
Specialist Medical ReviewerMichael Mallea, MD - Nephrology
Current as ofNovember 20, 2015