Creatinine and creatinine clearance blood tests
Creatinine is a waste product from the normal breakdown of muscle tissue. As creatinine is produced, it's filtered through the kidneys and excreted in urine. Doctors measure the blood creatinine level as a test of kidney function. The kidneys' ability to handle creatinine is called the creatinine clearance rate, which helps to estimate the glomerular filtration rate (GFR) - the rate of blood flow through the kidneys.
Normal kidney function and the GFR
All the blood in the body flows through the kidneys hundreds of times each day. The kidneys push the liquid part of blood through tiny filters (called nephrons), then reabsorb most of the fluid back into the blood. The fluid and waste products that the kidneys don't reabsorb are excreted as urine.
The rate of blood flow through the kidneys is the glomerular filtration rate, or GFR. (The glomeruli are microscopic bundles of blood vessels inside nephrons, and are crucial parts of the filtering system.) The glomerular filtration rate can't be measured directly - that's where measuring creatinine and creatinine clearance comes in.
What is creatinine and creatinine clearance?
Creatinine is a waste product that is produced continuously during normal muscle breakdown. The kidneys filter creatinine from the blood into the urine, and reabsorb almost none of it.
The amount of blood the kidneys can make creatinine-free each minute is called the creatinine clearance. This can vary depending on age, sex and size. Generally, the creatinine clearance is a good estimation of the glomerular filtration rate.
Measuring creatinine clearance and renal function
Doctors use tests to measure creatinine clearance as a check-up for renal function (kidney function). Testing the rate of creatinine clearance shows the kidneys ability to filter the blood. As renal function declines, creatinine clearance also goes down.
There are two main ways doctors use creatinine tests to measure kidney function:
- Creatinine clearance can be precisely determined by measuring the amount of creatinine present in a sample of urine collected over 24 hours. This method requires a person to urinate exclusively in a plastic container for one day, then bring it in for testing. Although the urine creatinine measurement method is inconvenient, it may be necessary to diagnose some kidney conditions.
- GFR can be estimated using a single blood level of creatinine, which your doctor enters into a formula. Different formulas are available, which take into account age, sex and sometimes weight and ethnicity. The higher the blood creatinine level, the lower the estimated GFR (eGFR) and creatinine clearance.
For practical reasons, the blood test estimation method for GFR is used far more often than the 24-hour urine collection test for creatinine clearance.
Understanding an abnormal creatinine test result
A low eGFR or creatinine clearance demonstrates kidney disease. The decline in kidney function can be either acute (sudden, often reversible) or chronic (long-term and irreversible). Repeated eGFR or creatinine clearance measurements over time can identify kidney disease as acute or chronic.