What Is a Creatinine Test?
A creatinine test, also called a serum creatinine test, is a way for doctors to measure how well your kidneys are working. Creatinine is a waste product from the normal breakdown of muscle tissue. As your body makes it, it's filtered through your kidneys and expelled in urine. Your kidneys' ability to handle creatinine is called the creatinine clearance rate, and this helps estimate how fast blood is moving through your kidneys, called the glomerular filtration rate (GFR).
Normal Kidney Function and the GFR
All the blood in your body flows through your kidneys hundreds of times each day. The kidneys push the liquid part of blood through tiny filters called nephrons and then reabsorb most of the fluid back into the blood. Fluid and waste products that the kidneys don't reabsorb are sent out of your body in your urine.
The rate of blood flow through your kidneys is the GFR. (The glomeruli are tiny bundles of blood vessels inside nephrons, and they’re crucial parts of the filtering system.) Your doctor can’t measure the GFR itself, so that's where creatinine and creatinine clearance come in.
What Is Creatinine and Creatinine Clearance?
Creatinine is a waste product that your body constantly makes during normal muscle breakdown. Your 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 95 milliliters (mL) per minute for women and 120 mL per minute for men. This means each minute, that person's kidneys clear 95 to 120 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.
Why Creatinine Tests Are Done
Doctors use creatinine and creatinine clearance tests to check how well your kidneys work. This is called renal function. Testing the rate of creatinine clearance shows the kidneys' ability to filter the blood. As renal function gets worse, creatinine clearance also goes down. Your doctor may also order a creatinine test if you have symptoms of kidney disease, including:
A loss of appetite
Tiredness and weakness
Changes in how much you pee
Swollen feet and ankles
Preparing for a Creatinine Test
Before you take a creatinine test, your doctor may tell you not to eat cooked meat for 24 hours. Studies show that it can raise your creatinine levels for a short time.
How Is a Creatinine Test Done?
There are two main ways doctors use creatinine tests to measure kidney function:
Urine tests. Creatinine clearance can be pinpointed by measuring the amount of creatinine in a sample of pee collected over 24 hours. For this method, you store all your urine in a plastic jug for one day and then bring it in for testing. This method is inconvenient, but it may be necessary to diagnose some kidney conditions.
Blood tests.Doctors can estimate GFR using a single blood level of creatinine, which they enter into a formula. Different formulas take into account your 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 method for GFR is used far more often than the 24-hour urine collection test for creatinine clearance. But urine collections may still be useful in people who have large muscle mass or a marked decrease in muscle mass.
Creatinine Test Results
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 permanent). Repeated GFR or creatinine clearance measurements over time can identify kidney disease as acute or chronic.
Kidney function and creatinine clearance go down with age. Fortunately, your kidneys have a huge reserve capacity. Most people can lose 30 percent to 40 percent of their renal function without major problems.
Doctors figure out 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)
Stage 5: GFR less than 15 (kidney failure, usually requiring dialysis)
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.
Abnormal creatinine test results don’t always mean kidney disease. Sometimes, they can be signs of:
A diet high in red meat
Side effects of certain medicines
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. Otherwise, you may need more testing to identify the cause of kidney disease.
Checking the GFR or creatinine clearance regularly 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 changes in renal function.
Because over-the-counter medications (especially medications for mild aches, pains and headaches), herbs, and supplements can all affect your kidneys, don’t take any of these without first discussing with your doctor.
Most people don’t need dialysis until GFR and creatinine clearance fall very low. But because kidney function naturally declines with age, it's important to take action early to keep as much as you can.