Test Overview

Creatinine and creatinine clearance tests measure the level of the waste product creatinine in your blood and urine. These tests tell how well your kidneys are working. The substance creatine is formed when food is changed into energy through a process called metabolism . Creatine is broken down into another substance called creatinine, which is taken out of your blood by the kidneys and then passed out of your body in urine. See a picture of the kidneys .

Creatinine is made at a steady rate and is not affected by diet or by normal physical activities. If your kidneys are damaged and cannot work normally, the amount of creatinine in your urine goes down while its level in your blood goes up.

Three types of tests on creatinine can be done:

Blood creatinine level

The blood creatinine level shows how well your kidneys are working. A high creatinine level may mean your kidneys are not working properly. 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.

Creatinine clearance test

A creatinine clearance test measures how well creatinine is removed from your blood by your kidneys. A creatinine clearance test gives better information than a blood creatinine test on how well your kidneys are working. A creatinine clearance test is done on both a blood sample and on a sample of urine collected over 24 hours (24-hour urine sample).

Blood urea nitrogen-to-creatinine ratio (BUN:creatinine)

The levels of blood creatinine and blood urea nitrogen (BUN) can be used to find the BUN-to-creatinine ratio. A BUN-to-creatinine ratio can help your doctor check for problems, such as dehydration, that may cause abnormal BUN and creatinine levels.

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. A blood urea nitrogen (BUN) test measures the amount of urea in your blood. Like creatinine, it can help your doctor see how well your kidneys are working.

Why It Is Done

A blood creatinine level or a creatinine clearance test is done to:

• See if your kidneys are working normally.
• See if your kidney disease is changing.
• See how well the kidneys work in people who take medicines that can cause kidney damage.
• See if severe dehydration is present. Dehydration generally causes BUN levels to rise more than creatinine levels. This causes a high BUN-to-creatinine ratio. Kidney disease or blockage of the flow of urine 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.

It is important to drink enough fluids during the 24-hour urine collection but do not drink coffee and tea. These are diuretics that cause your body to pass more urine.

How It Is Done

Collection of 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 to the site and then a bandage.

Collection of 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 to mark 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

Blood test

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.

Urine test

There is no pain while collecting a 24-hour urine sample.

Risks

Risks of a blood test

There is very little chance of a problem from having 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.

Urine test

There is no chance for problems while collecting a 24-hour urine sample.

Results

Creatinine and creatinine clearance tests measure creatinine levels in your blood and urine to 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.

Normal

Normal results may vary from lab to lab.

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).

High values

• High creatinine blood levels. High creatinine blood levels can mean serious kidney damage or disease is present. Kidney damage can be caused by a life-threatening infection, shock , cancer, or low blood flow to the kidneys. Other conditions that can cause high blood creatinine levels include blockage of the urinary tract (such as by a kidney stone ), heart failure , dehydration , excessive blood loss that causes shock, gout , or muscle conditions (such as rhabdomyolysis , gigantism, acromegaly , myasthenia gravis , muscular dystrophy , and polymyositis). Usually a high blood creatinine level means that the creatinine clearance value is lower than normal.
• High creatinine clearance. High creatinine clearance values can be caused by strenuous exercise, muscle injury (especially crushing injuries), burns, carbon monoxide poisoning , hypothyroidism , and pregnancy.
• High BUN-to-creatinine ratio. High BUN-to-creatinine ratios occur with sudden (acute) kidney failure, which may be caused by shock or severe dehydration. A blockage in the urinary tract (such as a kidney stone) can cause a high BUN-to-creatinine ratio. A very high BUN-to-creatinine ratio may be caused by bleeding in the digestive tract or respiratory tract .

Low values

• Low blood creatinine levels. 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. Low creatinine clearance levels can mean serious kidney damage is present. 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, dehydration, and liver disease ( cirrhosis ), can also cause low creatinine clearance levels.
• Low BUN-to-creatinine ratio. A low BUN-to-creatinine ratio may be associated 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, and the use of certain medications.

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

• Taking medicines, such as methyldopa, trimethoprim (Proloprim, Trimpex), vitamin C (ascorbic acid), cimetidine (Tagamet), some diuretics , and cephalosporin antibiotics, especially cefoxitin (Mefoxin). These affect the blood creatinine levels.
• Taking medicines, such as vitamin C (ascorbic acid), phenytoin (Dilantin), some cephalosporin antibiotics, captopril, aminoglycosides, trimethoprim (Proloprim, Trimpex), cimetidine (Tagamet), quinine, quinidine (Cardioquin, Quinaglute, Quinidex), procainamide, and the antifungal medication amphotericin B. These affect the creatinine clearance levels.
• Taking medicines, such as cimetidine (Tagamet), steroids, and tetracycline antibiotics. These can affect the BUN-to-creatinine ratio.
• Doing strenuous exercise 2 days before creatinine clearance test.
• Eating more than 8 oz (227 g) of meat, especially beef, in the 24 hours before a blood creatinine test and during a creatinine clearance urine test.

What To Think About

• A high blood creatinine level is generally seen with a low creatinine clearance level 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, or mature, your baby's kidneys are. This can be helpful if there is a chance your baby will be delivered early. A baby who has mature kidneys 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 whether kidney damage may be present, a BUN level is also measured. Other tests may also be done to check for kidney disease. For more information, see the medical test 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 to regularly check how well the kidneys are working.
• Diabetes experts recommend that blood creatinine levels be done every year for people with diabetes . The creatinine level is used to find the glomerular filtration rate, which shows how well the kidneys are working.
• The amount of creatinine in the blood depends partly on the amount of muscle tissue; blood 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 blood creatinine and sodium levels to help find the fractional excretion of sodium (FENA). This test can help your doctor see whether a problem with blood flow to the kidneys is caused by dehydration or shock or by damage to the kidneys themselves.

References

Other Works Consulted

• American Diabetes Association (2008). Standards of medical care in diabetes. Diabetes Care, 31(Suppl 1): S12–S54.
• 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, eds. (2004). Laboratory Tests and Diagnostic Procedures, 4th ed. Philadelphia: Saunders.
• Fischbach FT, Dunning MB III, eds. (2004). Manual of Laboratory and Diagnostic Tests, 7th ed. Philadelphia: Lippincott Williams and Wilkins.
• Pagana KD, Pagana TJ (2006). Mosby’s Manual of Diagnostic and Laboratory Tests, 3rd ed. St. Louis: Mosby.

Credits