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. Creatinine clearance in a healthy young person is about 125 milliliters per minute -- meaning each minute, that person's kidneys clear 125 mL of blood free of creatinine. The GFR 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 creatinine and creatinine clearance tests to check 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 jug 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 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. However, the use of 24-hour collections for creatinine clearance may still be useful in patients who have large muscle mass or a marked decrease in muscle mass.
Understanding an Abnormal Creatinine Test Result
A low GFR 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 GFR or creatinine clearance measurements over time can identify kidney disease as acute or chronic.
Kidney function and creatinine clearance naturally decline with age. Fortunately, the kidneys have a huge reserve capacity. Most people can lose well over half their renal function without symptoms or significant problems.
Doctors determine the severity of chronic kidney disease with a staging system that uses GFR:
Stage 1: GFR 90 or greater (normal kidney function)
Stage 2: GFR 60-89 (mild decline in kidney function)
Stage 3a: GFR 45 - 59 (mild to moderate decline in kidney function)
Stage 3b GFR 30 - 44 (moderate to severe decline in kidney function)
Stage 4: GFR 15-29 (severe decline in kidney function)
People over age 60 may have an apparently normal creatinine blood level, but still have a low GFR and creatinine clearance. The 24-hour urine collection method, or one of the GFR estimation formulas, can more accurately identify the decline in kidney function.
What to Do About a Low Creatinine Clearance
If you have a low GFR or creatinine clearance, your doctor will design an action plan with you to address the problem.
The main causes of chronic kidney disease are high blood pressure and diabetes. If you have these conditions, the first step is to get them under control with improved diet, exercise, and medications. If these conditions are not present, further testing may be needed to identify the cause of kidney disease.
Periodically checking the GFR or creatinine clearance allows you and your doctor to follow any decline in kidney function over time. Your doctor may need to make changes in your medications to adjust for any decline in renal function.
Because over-the-counter medications (especially medications for mild aches, pains and headaches), herbs and supplements can all affect your kidneys, do not take any of these without first discussing with your doctor.
Most people do not need dialysis until GFR and creatinine clearance fall very low. However, because kidney function naturally declines with age, it's important to take action early to preserve all the kidney function you can.