When muscles use energy, they release a waste product called creatinine into the blood. The kidneys then filter creatinine from the blood. From the kidneys, creatinine passes out of the body through the urinary tract. If the kidneys are not functioning normally, high amounts of creatinine remain in the blood while low amounts are passed out in the urine.

During a normal pregnancy, increased blood volume and kidney function cause an increase in the amount of creatinine filtered out of the blood and passed into the urine. A measurement of this amount is called the creatinine clearance value. If the kidneys are damaged by preeclampsia, the creatinine clearance value decreases because the kidneys are filtering less creatinine out of the blood. The amount of kidney damage can be estimated by the amount of decrease in the creatinine clearance value.

The creatinine clearance test requires a blood sample and a sample of all the urine collected for 24 hours (24-hour urine sample).

For more information, see the medical test Creatinine and Creatinine Clearance.