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 how doctors can estimate how well kidneys are filtering the blood, which is 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 glomeruli 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 filtering by the kidneys is called GFR. (The glomeruli are tiny bundles of blood vessels and other cells inside nephrons, and they’re the crucial parts of the filtering system.) Your doctor can’t measure the GFR itself, so that's where serum creatinine come in.
Your kidneys filter creatinine from the blood into the urine and reabsorb almost none of it. The serum creatinine is then used to estimate a GFR (eGFR) based on a formula.
Why Creatinine Tests Are Done
Doctors use serum creatinine and eGFR to check how well your kidneys work. As kidney function gets worse, GFR 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
Or frequently, the test is part of a routine laboratory panel that is often checked as part of a routine physical. Kidney function is often also checked if you are taking certain medications or if you have conditions such as heart disease, liver disease, diabetes, hypertension, or cancer.
How Is a Creatinine Test Done?
There are two main ways doctors use creatinine tests to measure kidney function:
Urine tests. GFR can be estimated 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 and is now often done anymore, 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 and sex, and older formulas may use race/ethnicity. The higher the blood creatinine level, the lower the estimated GFR.
For practical reasons, the blood test method for GFR is used far more often than the 24-hour urine collection test. 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 demonstrates kidney disease. The decline in kidney function can be either acute (sudden, often reversible) or chronic (long-term and permanent). Repeated GFR measurements over time can identify kidney disease as acute or chronic.
Kidney function and GFR 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
Stage 3a: GFR 45 - 59 (mild kidney disease)
Stage 3b GFR 30 - 44 (moderate kidney disease)
Stage 4: GFR 15-29 (advanced kidney disease)
Stage 5: GFR less than 15 (close to or at kidney failure, often requiring dialysis or kidney transplantation)
People over age 60 may have an apparently normal creatinine blood level but still have a low GFR. This opposite can also be true, where GFR may be mildly low but there is no real kidney disease. The 24-hour urine collection method, or another blood test called cystatin C, can more accurately categorize which of these individuals have kidney disease which do not.
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
Increased muscle mass
What to Do About a Low GFR
If you have a low GFR, 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.
If the cause of the kidney disease is unclear, or if the kidney disease is advanced, your doctor will likely refer you to a kidney specialist called a nephrologist.
Checking the GFR 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 kidney 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 them with your doctor.
Most people don’t need dialysis until GFR falls very low (such as below 10). But because kidney function naturally declines with age, it's important to take action early to keep as much as you can.