What is being tested?
Creatinine is produced in your muscles when a compound called creatine spontaneously breaks down. Creatine is used in a process in body cells to produce the energy needed to contract muscles and it is produced at a relatively constant rate. Almost all creatinine is excreted by the kidneys, so blood levels are a good measure of how well your kidneys are working.

How is the sample collected for testing?
A blood sample is drawn from a vein in the arm. You may be asked to collect a complete 24-hour urine sample in addition to having your blood taken (see "Creatinine Clearance"). Your doctor or the laboratory will give you a large container and instructions for properly collecting this sample. You will normally be asked to collect urine as soon as you wake up in the morning until the same time the following day.