To help detect and evaluate kidney dysfunction or decreased blood flow to the kidneys
Your doctor will request this test if he/she thinks that you may have a problem affecting how your kidneys work, such as a blockage within the kidney, damage to the kidneys, dehydration or fluid loss, or another disease, such as congestive heart failure. Creatinine clearance may also be measured before you are given certain drugs which rely on good kidney function in order to allow the drugs to be removed from the body. Another way of measuring how well your kidneys are working, commonly used across the UK, includes the “estimated glomerular function rate“ (eGFR) which is calculated from a single blood sample taken from a vein in your arm. This is an ‘estimation’ as there is no direct or simple way of measuring kidney function.
This test measures the concentration (amount per volume) of creatinine in both a sample of blood and a sample of urine from a 24-hour urine collection. Creatinine is a protein from muscle, made constantly by the body, filtered freely by the kidney (freely passed into the urine without obstacle) and excreted (removed from the body, in this case in the urine) daily. The results are used to work out how much creatinine has been cleared from the blood and passed into the urine. This number reflects how much blood is being passed throughthe filtering part of the kidneys (the glomeruli) in a 24-hour time period.
The amount of blood filtered per minute is known as the Glomerular Filtration Rate (GFR). If the glomeruli are damaged or diseased, or if blood circulation is slowed, then less creatinine will be removed from the blood and the GFR will drop. As creatinine levels tend to remain the same in a person a single creatinine measurement fromthe blood can be used to assess GFR when combined with the creatinine found in the urine collected over a period of time.
How is the sample collected for testing?
The test requires a 24-hour urine collection and a blood sample taken either at the beginning or end of the urine collection. The blood sample is collected by needle from a vein in the arm.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed but heavy exercise should be avoided to prevent falsely high readings. Protein supplements should also be avoided during the test as they contain creatinine. You should drink as normal with no other change to your routine or habits.
How is it used?
The creatinine clearance test is used to help evaluate the rate and efficiency of kidney filtration. It is used to help detect kidney dysfunction resulting from either kidney disease or decreased blood flow to the kidneys
When is it requested?
What does the test result mean?
An example of a normal creatinine clearance is 100 mL/minute. You may find it useful to consider your results in percentage terms. For example if your creatinine clearance is 50, then you have lost about 50% of your kidney function. This may be a permanent or temporary loss. A low creatinine clearance may indicate a drop in the amount of blood filtered by the kidney either due to disease within the kidney cells or to less blood getting to the kidneys. Congestive heart failure, diabetes, dehydration, shock (meaning losing a lot of fluid from the body for example if someone was to bleed after an accident or operation), blockage within the kidney, or acute or chronic kidney failure are among the possible causes.
Are there other ways to estimate or determine the Glomerular Filtration Rate (GFR) of my kidneys?
Is there anything else I should know?
What should I do if I forget to save one urine sample during the collection?
Is this test extremely accurate?
There are other, more involved tests that have higher accuracy. However, these are more complex for the patient and doctor and involve injecting a radioactive tracer in the patient. For most purposes, the accuracy of creatinine clearance is good enough. The estimated GFR produces very similar information, and does not require a urine collection. Creatinine clearance measurement is now rarely done except for some drug dose decisions (particularly for some chemotherapy) and in children (where eGFR calculations cannot be so accurate but usually only prior to giving specific drugs).
Can I perform this test at home?