Creatinine and Creatinine Clearance

Creatinine and Creatinine ClearanceTest Overview
Creatinine and creatinine clearance tests measure the level of the waste product creatinine in your blood and urine. These tests provide information about how well your kidneys are working. Creatine is a substance that forms when food is converted into energy through a process called metabolism. Creatine is broken down into another substance called creatinine, which is filtered out of your blood by the kidneys and then passed out of your body in urine. See an illustration of the kidneys.

Creatinine is produced at a steady rate and is affected very little by diet or by normal physical activities. If your kidneys are damaged and cannot function normally, the amount of creatinine in your urine decreases while its level in your blood increases.

Three types of tests involving creatinine can be done:
Blood creatinine level
The blood creatinine level indicates 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; therefore, 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. Compared to a blood creatinine level, a creatinine clearance test provides a more precise measure of how well your kidneys are working. A creatinine clearance test is performed both on a blood sample taken from your vein 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 calculate the BUN-to-creatinine ratio. A BUN-to-creatinine ratio can help your health professional predict which conditions, such as dehydration, may be causing abnormal BUN and creatinine levels and decreased kidney function.
Urea is a waste product formed when protein is broken down in your body. Urea is produced in the liver and eliminated from your body in urine. A blood urea nitrogen (BUN) test measures the amount of urea in your blood. Like creatinine, it can help your health professional predict how well your kidneys are functioning.
From the creatinine and BUN levels, a measurement called the BUN-to-creatinine ratio can be calculated. The BUN-to-creatinine ratio is determined by dividing the measured BUN level by the creatinine level.

Why It Is Done
A blood creatinine level or a creatinine clearance test is done to:

– Determine whether your kidneys are functioning normally.
– Monitor the progress of your kidney disease.
– Monitor the kidney function of people who take medications that can cause kidney damage.
– Determine whether severe dehydration is present. A BUN-to-creatinine ratio can help your health professional predict whether decreased kidney function is the result of dehydration or kidney disease. Dehydration usually causes BUN levels to rise more than creatinine levels, resulting in an increased BUN-to-creatinine ratio. Kidney disease or blockage of the flow of urine from the kidney usually causes both BUN and creatinine levels to increase to a similar degree, resulting in a BUN-to-creatinine ratio that is close to normal.

How To Prepare
- Avoid strenuous exercise 48 hours prior to having creatinine tests.
– Do not eat an excessive amount [more than 8 oz(227 g)] of meat, especially beef, or other protein for 24 hours prior to the tests and during a creatinine clearance test.
– It is important to drink enough fluids during the 24-hour urine collection but avoid coffee and tea which are substances that remove water from the body by promoting urine formation and the loss of salt (diuretics).

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.
– Apply a gauze pad or cotton ball over the needle site as the needle is removed.
– Apply pressure to the site and then a bandage.
Collection of the 24-hour urine sample
– The collection period usually starts 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 health professional or lab will usually provide you with a large container that holds about 1 gal(3.8 L) and has a small amount of preservative in it. Urinate into a smaller, clean container and then pour the urine into the large container. Avoid touching the inside of the container with your fingers.
– Keep the large container in the refrigerator during the collection period.
– Empty your bladder for the final collection at or just before the end of the 24-hour period. Add this final sample to the large container and record the time.
– Avoid getting toilet paper, pubic hair, stool (feces), menstrual blood, or other foreign matter in the urine sample.

How It Feels
Blood test
You may feel nothing at all from the needle puncture, or you may feel a brief sting or pinch as the needle goes through the skin. Some people feel a stinging pain while the needle is in the vein. However, many people do not feel any pain (or have only minor discomfort) once the needle is positioned in the vein. The amount of pain you feel depends on the skill of the health professional drawing the blood, the condition of your veins, and your sensitivity to pain.

Urine test
There is normally no discomfort involved with collecting a 24-hour urine sample.

Risks
Risks of a blood test

There is very little risk of complications from having blood drawn from a vein.
– You may develop a small bruise at the puncture site. You can reduce the risk of bruising by keeping pressure on the site for several minutes after the needle is withdrawn.
– Rarely, the vein may become inflamed after the blood sample is taken. This condition is called phlebitis and is usually treated with a warm compress applied several times daily.
– Continued bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medications can also make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medication, tell your health professional before your blood is drawn.

Urine test
There are no risks associated with collecting a 24-hour urine sample.

Results
Creatinine and creatinine clearance tests measure creatinine levels in your blood and urine to provide information about how well your kidneys are working. The creatinine clearance value is calculated from the amounts of creatinine in the urine and blood and from the volume of urine. This value is reported as the amount of blood cleared of creatinine per minute, adjusted for your size.

Normal
Normal results may vary from lab to lab.
Blood creatinine and creatinine clearance

Blood creatinine: 0.7–1.2 milligrams per deciliter (mg/dL)
Creatinine clearance: 90–140 milliliters per minute (mL/min)

Creatinine clearance values normally drop as you get older (normal values usually drop by 6 mL/min for every 10 years past the age of 20).

BUN-to-creatinine ratio

Over 12 months of age: 10:1–20:1
Infants less than 12 months of age: Up to 30:1


High values

High creatinine blood levels. High creatinine blood levels can indicate serious kidney damage or disease. Kidney damage can be caused by a life-threatening infection, ·          shock, cancer, or reduced blood flow to the kidneys. Other conditions that can cause increased 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, polymyositis). Usually a high blood creatinine level means that the creatinine clearance value is below 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 conditions such as shock or severe dehydration. An obstruction in the urinary tract (such as from a kidney stone) can cause an elevated 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 indicate a decrease in muscle mass caused by a disease, such as muscular dystrophy, or by aging. Low levels can also indicate 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 results indicate serious kidney damage. Kidney damage can be due to conditions such as a life-threatening infection, shock, cancer, reduced 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 results.
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
Factors that can interfere with your test and the accuracy of the results include the following:
Blood creatinine levels can be affected by several medications, including methyldopa (Aldomet), trimethoprim (Proloprim, Trimpex), vitamin C (ascorbic acid), cimetidine (Tagamet), some ·       diuretics, and cephalosporin antibiotics, especially cefoxitin (Mefoxin).
Creatinine clearance results can be affected by some medications, including vitamin C (ascorbic acid), phenytoin (Dilantin), some cephalosporin antibiotics, captopril, aminoglycosides (Garamycin), trimethoprim (Proloprim, Trimpex), cimetidine (Tagamet), quinine, quinidine (Cardioquin, Quinaglute, Quinidex), procainamide (Procan, Pronestyl), and the antifungal medication amphotericin B.
BUN-to-creatinine ratio results can be affected by several medications, including cimetidine (Tagamet), steroids, and tetracycline antibiotics.
–  Strenuous exercise should be avoided 48 hours prior to a creatinine clearance test.
–  Eating large amounts [more than 8 oz(227 g)] of meat, especially beef, in the 24 hours prior to and during a creatinine clearance test
–  Failing to collect exactly 24 hours of urine during a creatinine clearance test

What To Think About
– A high blood creatinine level is usually associated with a low creatinine clearance because creatinine in the blood is removed, or filtered, by the kidneys. If the kidneys are not able to filter creatinine properly from the blood (low creatinine clearance), levels of creatinine in the blood increase (high blood creatinine level).
– The amount of creatinine in amniotic fluid can be measured to estimate the maturity of a baby (fetus) and its ability to breathe if delivered. A baby with mature kidneys (and equally mature lungs) will produce more creatinine than an immature baby.
– A normal blood creatinine level does not rule out kidney disease. To help determine whether kidney damage may be present, a BUN level is also measured. Other tests may also need to be done to evaluate 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 usually indicates chronic kidney problems.
– The amount of creatinine in the blood depends partly on the amount of muscle tissue; therefore, 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 calculate a result called the fractional excretion of sodium (FENA). This test can help your health professional determine whether a problem with blood flow to the kidneys is caused by dehydration or shock versus damage to the kidneys themselves.

Credits

Author Jan Nissl, RN, BS
Editor Susan Van Houten, RN, BSN, MBA
Associate Editor Tracy Landauer
Primary Medical Reviewer Patrice Burgess, MD
- Family Medicine
Specialist Medical Reviewer D C Mendelssohn, MD, FRCPC
- Nephrology
Last Updated September 20, 2004

http://www.webmd.com/hw/lab_tests/hw4322.asp

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