The Creatinine Test
Your doctor may recommend you to take the creatinine test to assess how well your kidneys are functioning. This test may be done individually or along with other tests such as blood urea nitrogen (BUN) that are used to measure kidney function.
One of the main roles of the kidneys is to filter waste from your blood. Creatinine is a waste product that is generated by the body as part of the energy-producing process in the muscles during the breakdown of creatine phosphate. The kidneys should filter this creatinine out of the blood, and excrete it as a waste product in urine.
When the kidneys are not functioning well, creatinine builds up in the blood. Thus, a measurement of the creatinine in your blood gives an indication of how well the kidneys are working.
Creatinine levels may also be measured in the urine. This is done using a timed urine sample, typically over a 24-hour period. This means that all the urine passed during this time period is collected in a container and tested. The result shows how much creatinine has passed through your kidneys into your urine during this time period.