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The Test Sample
What is being tested?
Creatinine is a waste product produced by muscles from the breakdown of a compound called creatine. Creatinine is filtered from the blood by the kidneys and released into the urine. A creatinine clearance test measures creatinine levels 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.
The amount of creatinine produced in the body is dependent on muscle mass and is relatively constant for an individual. The amount of creatinine removed from the blood depends on both the filtering ability of the kidneys and the rate at which blood is carried to the kidneys.
The amount of blood filtered per minute by the kidneys is known as the glomerular filtration rate (GFR). If the kidneys 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.
GFR is difficult to measure directly. Therefore, it is recommended to estimate GFR by measuring the creatinine level in the blood and using the results in an equation to calculate estimated GFR. The calculation that takes into account several factors, such as age, gender and race of the person tested (see the article on Estimated Glomerular Filtration Rate).
Another, less common way to estimate GFR is to calculate creatinine clearance. There are several versions of the creatinine clearance calculation. All of them include the measurement of the amount of creatinine in a blood sample collected just before or after the urine collection, the amount of creatinine in a 24-hour urine sample, and the 24-hour urine volume. Since the amount of creatinine produced depends on muscle mass, some calculations also use a correction factor that takes into account a person's body surface area (using their height and weight).
How is the sample collected for testing?
The test requires a 24-hour urine collection and a blood sample drawn either at the beginning or end of the urine collection. The blood sample is drawn by needle from a vein in the arm. The person being tested will also usually be asked to provide their current height and weight.
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?
You may be instructed to fast overnight or refrain from eating cooked meat. Some studies have shown that eating cooked meat prior to testing can temporarily increase the level of creatinine.