Creatinine and creatinine clearance tests measure the level of the waste product creatinine in your blood and urine. These tests tell how well your kidneys are working. The substance creatine is formed when food is changed into energy through a process called metabolism. Creatine is broken down into another substance called creatinine, which is taken out of your blood by the kidneys and then passed out of your body in urine. See an illustration of the kidneys.

Creatinine is made at a steady rate and is not affected by diet or by normal physical activities. If your kidneys are damaged and cannot work normally, the amount of creatinine in your urine goes down while its level in your blood goes up.

Three types of tests on creatinine can be done:

Blood creatinine level

The blood creatinine level shows how well your kidneys are working. A high creatinine level may mean your kidneys are not working properly. The amount of creatinine in the blood depends partly on the amount of muscle tissue you have; men generally have higher creatinine levels than women.

Creatinine clearance test

A creatinine clearance test measures how well creatinine is removed from your blood by your kidneys. A creatinine clearance test gives better information than a blood creatinine test on how well your kidneys are working. A creatinine clearance test is done on both a blood sample and on a sample of urine collected over 24 hours (24-hour urine sample).

Blood urea nitrogen-to-creatinine ratio (BUN:creatinine)

The levels of blood creatinine and blood urea nitrogen (BUN) can be used to find the BUN-to-creatinine ratio. A BUN-to-creatinine ratio can help your doctor check for problems, such as dehydration, that may cause abnormal BUN and creatinine levels.

Urea is a waste product made when protein is broken down in your body. Urea is made in the liver and passed out of your body in the urine. A blood urea nitrogen (BUN) test measures the amount of urea in your blood. Like creatinine, it can help your doctor see how well your kidneys are working.

Why It Is Done

A blood creatinine level or a creatinine clearance test is done to:

• See if your kidneys are working normally.
• See if your kidney disease is changing.
• See how well the kidneys work in people who take medicines that can cause kidney damage.
• See if severe dehydration is present. Dehydration generally causes BUN levels to rise more than creatinine levels. This causes a high BUN-to-creatinine ratio. Kidney disease or blockage of the flow of urine from your kidney causes both BUN and creatinine levels to rise.