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.

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.

If the collection is being taken from an infant, a couple of extra collection bags may be necessary.

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.

The creatinine clearance is an estimate 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 the ability of the glomeruli to act as filters.

Because a small amount of creatinine is secreted by the kidney tubules, creatinine clearance is not exactly equivalent to the GFR. In fact, creatinine clearance usually overestimates the GFR. This is particularly true in patients with advanced renal failure, where the percentage of secreted creatinine in the urine means a greater percentage of the overall urine creatinine.

Normal values are as follows (normal value ranges may vary slightly among different laboratories):

• Male: 97 to 137 ml/min.
• Female: 88 to 128 ml/min.

Note: ml/min. = milliliters per minute

Abnormal results are lower-than-normal GFR measurements, and they may indicate:

• Acute tubular necrosis
• Congestive heart failure
• Dehydration
• Glomerulonephritis
• Renal ischemia (blood deficiency)
• Shock
• Acute bilateral obstructive uropathy
• Acute nephritic syndrome
• Acute renal failure
• Chronic renal failure
• End-stage renal disease
• Rapidly progressive (crescentic) glomerulonephritis
• Wilms' tumor

The risks of the test are minimal and are related to the blood draw process. Rarely, the following will occur:

• Excessive bleeding at blood draw site
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins

Factors that may interfere with the accuracy of the test are as follows:

• Incomplete urine collection
• Pregnancy or vigorous exercise

Drugs that can interfere with creatinine clearance measurements include: cimetidine, trimethoprim, and nephrotoxic drugs, such as cephalosporins (cefoxitin).

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.