This test measures how well your kidneys are working and how well blood is flowing to them. Creatinine is a waste product that comes from normal muscle use and from meat protein in your diet. Healthy kidneys remove creatinine from the blood so it can be eliminated from your body through urine.
The creatinine clearance test usually compares the creatinine level in a 24-hour urine sample with the creatinine level in your blood to evaluate how well your kidneys are working. Creatinine clearance also helps your doctor estimate your glomerular filtration rate (GFR). This is the amount of blood cleaned by tiny kidney filters called glomeruli each minute.
You may need this test if you have symptoms that might be caused by kidney disease. Signs and symptoms include:
Pain near the kidneys
Swelling or puffiness, especially around the eyes and ankles
High blood pressure
Low amount of urine or trouble urinating
Dark or foamy urine
Blood in the urine
Pain in the midback
You may also need this test if the results of other blood or urine tests suggest that you may have a problem with your kidneys.
If you have already been diagnosed with kidney disease, your doctor can use this test regularly to watch your kidney function and adjust your treatment if needed. If you have diabetes, congestive heart failure, or another disease that affects the kidneys, you may have a creatinine clearance test to look for any changes in your kidney function.
You may also have a blood urea nitrogen (BUN) test. If your BUN level is high, it's a sign that your kidneys may not be working properly.
You may also have other tests to measure how much protein is leaking from your kidneys into your urine.
Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.
Results are given in milliliters per minute (mL/min). The range for a normal test result depends on your age and gender. For adults younger than 40, normal levels are typically in the range of:
107 to 139 mL/min or 1.78 to 2.32 mL/s (SI units) for men
87 to 107 mL/min or 1.45 to 1.78 mL/s (SI units) for women
Creatinine clearance rates go down as you age. For every decade after age 40, a normal test result is 6.5 mL/min less than the numbers above.
For newborns, the normal range is 40 to 65 mL/min.
An abnormal creatinine clearance rate may mean you have a problem with your kidneys, or it may mean a problem somewhere else in your body is affecting blood flow to the kidneys.
This test requires samples of blood and urine. The blood sample is drawn through a needle from a vein in your arm.
For a 24-hour urine test, you must collect all of your urine for 24 hours. Empty your bladder completely first thing in the morning without collecting it and note the time. Then collect your urine every time you go to the bathroom over the next 24 hours, including the following morning within 10 minutes of the time you collected your first urine sample.
Follow your health care provider's instructions for storing the urine. Keep it out of bright light and store it in a cool place, like the refrigerator.
Giving a urine sample poses no known risks.
Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.
The 24-hour urine sample has to be exact, meaning all urine must be collected and properly stored during the test period. Otherwise, the test results may not be accurate. This is a common problem with the creatinine clearance test, so many doctors use a formula based on your blood creatinine levels alone to estimate the GFR.
If you are pregnant or have recently done intense exercise, such as running a marathon, your creatinine clearance rate could be higher than normal.
Certain medications can cause creatinine clearance levels to be lower than normal. These include cimetidine, procainamide, certain antibiotics, and quinidine. Certain conditions, such as diabetes and congestive heart failure, may also affect the results.
You don't need to prepare for this test. Be sure your doctor knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use.