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.
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.
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 the same as the GFR. In fact, creatinine clearance usually overestimates the GFR. This is particularly true in patients with advanced kidney disease.
Clearance is often measured as milliliters/minute (ml/min). Normal values are:
- Male: 97 to 137 ml/min.
- Female: 88 to 128 ml/min.
The examples above are common measurements for results of these tests. Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your doctor about the meaning of your specific test results.
What abnormal results mean
Abnormal results (lower than normal creatinine clearance) may indicate:
- Acute tubular necrosis
- Bladder outlet obstruction
- Congestive heart failure
- End-stage kidney disease
- Kidney failure
- Renal ischemia (too little blood flow to the kidneys)
- Renal outflow obstruction (usually must affect both kidneys to reduce the creatinine clearance)
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 lightheaded
- 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
- Vigorous exercise
Drugs that can interfere with creatinine clearance measurements include: cimetidine, trimethoprim, and drugs that can damage the kidneys, such as cephalosporins.
Landry DW, Bazari H. Approach to the patient with renal disease. In: Goldman L, Schafer AI, eds. . 24th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 116.