Creatinine clearance

Definition

The creatinine clearance test compares the level of creatinine in urine with the creatinine level in the blood. (Creatinine is a breakdown product of creatine, which is an important part of muscle.) The test helps provide information on kidney function.

See also:

• Creatinine - blood test
• Creatinine - urine test

How the test is performed

This test requires both a blood and urine sample. You will collect your urine for 24 hours, and then have blood taken.

• For information on giving a blood sample, see: venipuncture.
• For information on how to collect the urine sample, see: 24-hour urine collection.

The samples are sent to a laboratory. The laboratory specialist measures the level of creatinine in both the urine and blood samples, and looks at how much urine you collected in 24 hours.

The clearance rate is then calculated. The calculation is adjusted for your specific 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).

How to prepare for the test

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

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 test is used to estimate the glomerular filtration rate (GFR).

However, because a small amount of creatinine is released by the filtering tubes in the kidneys, creatinine clearance is not exactly thto the same as the GFR. In fact, creatinine clearance usually overestimates the GFR. This is particularly true in patients with advanced kidney failure.

Normal Values

Clearance is often measured as milliliters/minute (ml/min). Normal values are:

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

Note: Normal values ranges may vary slightly among different laboratories.

What abnormal results mean

Abnormal results (lower-than-normal creatinine clearance) may indicate:

• Acute tubular necrosis
• Bladder outlet obstruction
• Congestive heart failure
• Dehydration
• Glomerulonephritis
• Renal ischemia (blood deficiency
• Renal outflow obstruction (usually must affect both kidneys to reduce the creatinine clearance)
• Shock
• Acute renal failure
• Chronic renal failure
• End-stage renal disease

What the risks are

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

Special considerations

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

• Incomplete urine collection
• Pregnancy
• Vigorous exercise

Drugs that can interfere with creatinine clearance measurements include: cimetidine, trimethoprim, and drugs that can damage the kidneys, such as cephalosporins.

The creatinine clearance test should only be done for patients who are medically stable. Such patients may have a rapidly changing creatinine clearance, and therefore any result may be inaccurate.

References

Bazari H. Approach to the patient with renal disease. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 115.