At a Glance
Why Get Tested?
To help detect and evaluate kidney dysfunction or decreased blood flow to the kidneys
When to Get Tested?
If your doctor thinks that you may have a problem affecting the function of your kidneys, such as an obstruction within the kidney, acute or chronic kidney failure, dehydration, shock, or another disease, such as congestive heart failure. Creatinine clearance may also be measured before you are prescribed certain drugs which rely on good kidney function in order to allow the drugs to be removed from the body effectively after they have carried out their purpose. Other straightforward methods of estimating the function of your kidneys now in routine use across the UK include the calculation of estimated glomerular function rate (eGFR) from a single blood sample taken from a vein in your arm.
Both a urine sample (24-hour collection) and a blood sample taken from a vein in your arm.
Test Preparation Needed?
The Test Sample
What is being tested?
This test measures the concentration of creatinine in both a sample of blood and a sample of urine from a 24-hour urine collection. The results are used to calculate the amount of creatinine that has been cleared from the blood and passed into the urine. This calculation allows for a general evaluation of the amount of blood that is being filtered by the kidneys in a 24-hour time period.
Creatinine is a chemical derived from creatine, a nitrogen-based organic compound used by muscles to store and transfer energy. The amount of creatinine produced in the body is dependent on muscle mass and is relatively constant for an individual.
Almost all creatinine is removed from the body as blood passes through the kidneys. The amount of creatinine removed from the blood depends on both the filtering ability of the glomeruli in the kidneys and the rate at which blood is carried to the kidneys.
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 released into the urine and the GFR will be decreased. By measuring creatinine in the blood and in the urine, it is possible to determine the amount of blood filtered by the kidneys in a measured period of time. A calculation is made based on the amount of creatinine in the urine, the time period of collection, and the amount of creatinine in the blood.
Since the amount of creatinine produced depends on muscle mass, in some circumstances, e.g. in children, the calculation will need to be corrected for body surface area.
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.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
Ask a Laboratory Scientist
NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.