Creatinine clearance
Alternative names:
GFR; glomerular filtrate rate
How the test is performed:
A 24-hour urine sample is needed.
The health care provider will instruct you, if necessary, to discontinue drugs that may interfere with the test. - On day 1, urinate into the toilet upon arising in the morning.
- Collect all subsequent urine (in a special container) for the next 24-hours.
- On day 2, urinate into the container in the morning upon arising.
- Cap the container. Keep it in the refrigerator or a cool place during the collection period. Label the container with your name, the date, the time of completion, and return it as instructed.
Infant:
Thoroughly wash the area around the urethra. Open a urine collection bag (a plastic bag with an adhesive paper on one end), and place it on your infant. For males, the entire penis can be placed in the bag and the adhesive attached to the skin. For females, the bag is placed over the labia. Place a diaper over the infant (bag and all). The infant should be checked frequently and the bag changed after the infant has urinated into the bag. For active infants, this procedure may take a couple of attempts--lively infants can displace the bag, causing an inability to obtain the specimen. The urine is drained into the container for transport to the laboratory.
Deliver it to the laboratory or your health care provider as soon as possible upon completion.
Adult or child:
Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill with blood). A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the tourniquet is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.
Infant or young child:
The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.
Both the blood and urine will be tested in a laboratory.
How to prepare for the test:
Infants and children:
If the collection is being taken from an infant, a couple of extra collection bags may be necessary.
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experience, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:
How the test will feel:
The urine test involves only normal urination and there is no discomfort. When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
Why the test is performed:
The creatinine clearance is a measure of the glomerular filtration rate, that is, the volume of filtrate made by the kidneys per minute. The urine and serum creatinine levels are measured along with the urine volume in 24-hours; the clearance rate is then calculated. The calculation uses a correction factor for body size. The creatinine clearance appears to decrease with age (each decade corresponds to a decrease of about 6.5 ml/min./1.73 m2).
Creatinine is used for this purpose since it is normally present in the body, and very little creatinine is reabsorbed after it is filtered. The amount of filtrate made in the kidney depends on the amount of blood that passes through the glomeruli and on the ability of the glomeruli to act as filters.
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