Creatinine clearance Health Article

Definition

The creatinine clearance test compares the level of creatinine in urine with the creatinine level in the blood, usually based on measurements of a 24-hour urine sample and a blood sample drawn at the end of the 24-hour period. Clearance is often measured as milliliters/minute (ml/min).

Because creatinine is found in stable plasma concentrations, is freely filtered and not reabsorbed, and is minimally secreted by the kidneys, creatinine clearance is used to estimate the glomerular filtration rate (GFR) -- the standard by which kidney function is assessed.

How the test is performed

A 24-hour urine sample generally required. Occasionally, 6- or 12-hour urine collections can be done alternatively. 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 when you get up in the morning.
• Afterwards, collect all urine in a special container for the next 24 hours.
• On day 2, urinate into the container when you get up in the morning.
• 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.

For an 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 the infant. For boys, the entire penis can be placed in the bag and the adhesive attached to the skin. For girls, the bag is placed over the labia. Diaper as usual over the secured bag.

This procedure may take a couple of attempts -- lively infants can displace the bag. The infant should be checked frequently and the bag changed after the infant has urinated into the bag. 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.

For an adult or child:

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic. An elastic band is placed around the upper arm to apply pressure and cause the vein to swell 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 band 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.

For an 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. A bandage may be applied to the puncture site if there is any bleeding.

Both the blood and urine will be tested in a laboratory.