To assess kidney function and diagnose, stage, and monitor chronic kidney disease (CKD)
Estimated Glomerular Filtration Rate (eGFR)
With a blood creatinine test as part of a routine health exam or when your healthcare practitioner thinks that you may have kidney damage or that you may be at risk for developing kidney disease; if you are diagnosed with CKD, testing is recommended at least once a year.
A blood sample drawn from a vein; the eGFR is a calculated estimate of the actual glomerular filtration rate and is based on your blood creatinine level and/or blood cystatin C level. Depending on the formula used, your age, sex, and race may also be needed.
This test typically uses a blood creatinine level to calculate a result. You may be instructed to fast overnight or refrain from eating cooked meat; some studies have shown that eating cooked meat prior to testing can temporarily increase the level of creatinine.
Glomerular filtration rate (GFR) is a measure of how well your kidneys are working. The kidney's primary function is to filter blood. Waste and excess water gets removed and turned into urine. The levels of salts and minerals in blood are adjusted to maintain a healthy balance. In addition, kidneys produce hormones that regulate blood pressure, maintain bone health, and control production of red blood cells.
Glomeruli are tiny filters in the kidneys that allow waste products to be removed from the blood, while preventing the loss of important substances, including proteins and blood cells. Every day, healthy kidneys filter about 200 quarts of blood and produce about 2 quarts of urine. The GFR refers to the amount of blood that is filtered by the glomeruli per minute. As kidney function declines due to damage or disease, the filtration rate decreases and waste products begin to build up in the blood.
Chronic kidney disease (CKD) is a common disease associated with a slow and progressive decrease in kidney function. Diabetes and high blood pressure are the two main causes of CKD. Aging is another risk factor for CKD that is often overlooked. As we age, so do our kidneys, losing about 1% of kidney filtration per year after age of 40.
Kidney diseases tend to progress silently and most cases of early stages of CKD go undiagnosed. Most people have no symptoms until 30-40% of kidney function is lost. Early detection of kidney dysfunction is crucial because it can help to minimize the damage by starting a suitable treatment. Screening for CKD is recommended for high risk groups, such as people with diabetes, high blood pressure (hypertension), heart disease, family history of kidney failure, and the elderly (age 60 and older).
Simple laboratory tests used to evaluate kidney function include the blood creatinine test and/or blood cystatin C test to estimate GFR and urine albumin and urine creatinine clearance.
Measuring glomerular filtration rate (GFR) directly is considered the most accurate way to detect changes in kidney function, but measuring the GFR directly is complicated, requires experienced personnel, and is typically performed only in research settings and transplant centers. For this reason, the estimated GFR (eGFR), which represents the best routinely available measurement of kidney function, is usually used. See "How can my actual GFR be determined?" under Common Questions for more information.
The eGFR is calculated according to the formulae recommended by the National Kidney Foundation using measured test results of blood creatinine levels and in special circumstances blood cystatin C levels.
- The most common approach is the eGFR based on blood creatinine level along with other variables such as age, sex, and race (e.g., African-American, non-African American), depending on the equation used. Creatinine is a muscle waste product that is filtered from the blood by the kidneys and released into the urine at a relatively steady rate. When kidney function decreases, less creatinine is eliminated and levels increase in the blood. The variations in muscle metabolism and muscle mass can lead to significant differences in measured creatinine levels. The eGFR may not be reliable in amputees, body builders, vegetarians, patients at the extremes of weight and age, and those with rapidly changing kidney function (e.g., acute kidney injury). In addition, some medications can increase creatinine.
- The eGFR calculated based on blood cystatin C level is recommended for confirmatory testing when eGFR based on serum creatinine is less accurate (e.g., elderly, overweight or people with abnormal muscle mass). Cystatin C is a small protein produced at a constant rate by all nucleated cells in the body and is found in various body fluids, including blood. It is filtered by the kidneys and broken down at a constant rate. It does not reenter the blood circulation or get eliminated in the urine. Unlike creatinine, which is affected by muscle mass, cystatin C is affected by age, inflammation, obesity, and diabetes. Cystatin C is a relatively new biomarker and extensive research is ongoing to define its optimal use. See "Is there anything else I should know?" under Common Questions for more information.
Different equations may be used to calculate eGFR. The following two are most common and require a blood creatinine result, age, and assigned values based upon sex and race.
- Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) creatinine equation (2009)recommended by the National Kidney Foundation for calculating eGFR in adults.
- Modification of Diet in Renal Disease Study (MDRD) equationsome laboratories continue to use this equation.
Additional sets of CKD-EPI equations recommended by the National Kidney Foundation for adults that use cystatin C level to calculate eGFR, along with age and gender, include:
- Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) cystatin C equation (2012)
- Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) creatinine and cystatin C equation (2012)
The results reported using one of the above equations versus the other will not be identical but should give a healthcare practitioner similar information.
Separate equations, based on serum creatinine levels and height, are recommended for use in youth age 18 years and under. One is the updated Schwartz equation that takes the blood urea nitrogen (BUN) level into consideration.
The eGFR equations are not valid for those who are 70 years of age or older because muscle mass normally decreases with age.
See "Could I calculate my own eGFR?" under Common Questions for more information about these eGFR equations.
- How is the test used?
The estimated glomerular filtration rate (eGFR) is used to screen for and detect early kidney damage, to help diagnose chronic kidney disease (CKD), and to monitor kidney status.
The National Kidney Disease Education Program, American Society of Nephrology, and the National Kidney Foundation all recommend that an eGFR be calculated every time a creatinine blood test is done. The creatinine test is ordered frequently as part of a routine comprehensive metabolic panel (CMP) or basic metabolic panel (BMP), or along with a blood urea nitrogen (BUN) test to evaluate the health of your kidneys.
Creatinine, along with eGFR, is often used to monitor people with known CKD and those with conditions such as diabetes and high blood pressure (hypertension) that may lead to kidney damage.
The eGFR calculated using cystatin C is used similarly to creatinine-based eGFR. Cystatin C is not a part of the routine testing and may not be offered by all clinical laboratories. Your healthcare professional will decide if you may benefit from this additional testing.
- When is it ordered?
A creatinine test and eGFR may be ordered when a healthcare practitioner wants to evaluate your kidney function as part of a health checkup or if kidney disease is suspected. Signs and symptoms of kidney disease may include:
- Swelling or puffiness, particularly around the eyes or in the face, wrists, abdomen, thighs, or ankles
- Urine that is foamy, bloody, or coffee-colored
- A decrease in the amount of urine
- Problems urinating, such as a burning feeling or abnormal discharge during urination, or a change in the frequency of urination, especially at night
- Mid-back pain (flank), below the ribs, near where the kidneys are located
- High blood pressure (hypertension)
As kidney disease worsens, symptoms may include:
- Urinating more or less often
- Feeling itchy
- Tiredness, loss of concentration
- Loss of appetite, nausea and/or vomiting
- Swelling and/or numbness in hands and feet
- Darkened skin
- Muscle cramps
An eGFR may be repeated if the initial result is abnormal to see if it persists.
The test is usually ordered periodically when you have chronic kidney disease (CKD) or a condition such as diabetes or hypertension that is associated with an increased risk of kidney damage.
- What does the test result mean?
Estimated GFR results are reported as milliliters/minute/1.73m2 (mL/min/1.73m2). Because some laboratories do not collect information on a patient's race when the sample is collected for serum creatinine testing, they may report calculated results for both African Americans and non-African Americans. The healthcare practitioner uses the result that applies to you in order to interpret the results correctly.
A normal eGFR for adults is greater than 90 mL/min/1.73m2, according to the National Kidney Foundation. (Clinical laboratories that use the MDRD equation to calculate eGFR will only report actual values that are less than 60 mL/min/1.73m2 because MDRD works best for estimating reduced kidney function. Clinical laboratories that use CKD-EPI equations often report all numerical values.)
An eGFR below 60 mL/min/1.73m2 suggests that some kidney damage has occurred. The test may be repeated to see if the abnormal result persists. Chronic kidney disease is diagnosed when you have an eGFR less than 60 mL/min/1.73m2 for more than three months.
You may have some kidney damage even with an eGFR greater than 90 mL/min/1.73m2. Other evidence, such as increased urine albumin, may indicate some degree of kidney damage. Thus, your eGFR should be interpreted in relation to your medical history and signs and symptoms.
The following table summarizes estimated GFR and the stages of kidney damage:
Kidney damage stage description estimated gfr† other findings 1 Normal or minimal kidney damage with normal GFR 90+ Protein or albumin in urine may be high, cells or casts rarely seen in urine (see Urinalysis) 2 Mild decrease in GFR 60-89 Protein or albumin in urine may be high, cells or casts rarely seen in urine 3 Moderate decrease in GFR 30-59 High blood pressure, anemia, abnormal serum calcium, potassium, phosphate 4 Severe decrease in GFR 15-29 High blood pressure, anemia, bone disease (CKD-MBD*) with abnormal serum calcium, phosphorous, vitamin D, parathyroid hormone 5 Kidney failure <15
† mL/min/1.73m2 * CKD-MBD: Chronic Kidney Disease-Mineral and Bone Disorder
- What other tests may be done to evaluate kidney function?
Other tests that may be done to help detect kidney damage and/or evaluate kidney function are:
- Urine albumin (microalbumin) and albumin/creatinine ratio (ACR)—used to screen people with chronic conditions, such as diabetes and hypertension, that put them at an increased risk of developing kidney disease; increased levels of albumin in the urine may indicate kidney damage.
- Urinalysis—may be used to help detect signs of kidney damage, such as the presence of blood or casts in the urine
- Renal panel—a group of tests that may be used to evaluate kidney function and/or screen for, diagnose or monitor kidney disease
The creatinine clearance test also provides an estimate of kidney function and of the actual GFR. However, in addition to the serum creatinine, this test requires a timed urine collection (24 hours) for urine creatinine measurement in order to compare blood and urine creatinine concentrations and to calculate the clearance.
- How can my actual GFR be determined?
The best method for directly determining the GFR is a procedure called an "inulin clearance." It involves introducing a fluid containing the marker molecule inulin (NOT insulin) into your veins (IV – intravenous infusion) and then collecting timed urines over a period of hours. The urine volumes are noted and the inulin in each sample is measured to allow determination of the GFR. This test and other methods of determining GFR, such as those that use radioactive markers, are not routinely ordered and are primarily performed in research settings.
- Could I calculate my own eGFR?
If you have had a recent creatinine or cystatin C measurement, you can calculate the eGFR by using one of the calculators for people 19 years of age or older on the National Kidney Foundation (NKF) website. If you have questions about the interpretation of your results, it is best to consult with your healthcare provider. For children and teens younger than 19, see the pediatric eGFR calculator on the NKF website.
The tables below summarize eGFR equations currently in use.
ADULT eGFR EQUATIONS (19 and over)
Equation Biomarker Recommended Comments CKD-EPI (2009) Creatinine KDIGO, NIDDK, NKF
- currently recommended for use in adults
- corrected for age, race and sex
- can be applied across all eGFR range, superior to MDRD in patients with normal or mildly reduced eGFR
- not validated in pregnant women, age groups below 18 and above 85, and certain ethnic groups
MDRD (1999) Creatinine NIDDK
- no longer recommended for use in adults
- corrected for age, race and sex
- can only be applied for eGFR less than 60
- was validated in CKD patients who were predominantly Caucasian, non-diabetic, and did not have kidney transplant
CKD-EPI (2012) Cystatin C KDIGO, NKF
- recommended for use in adults with unreliable creatinine levels (e.g., high or low muscle mass)
- numerous studies are underway to determine utility of new biomarker, cystatin C and corresponding eGFR
- at this time, there is no strong evidence to suggest that its performance is superior to creatinine CKD-EPI in all populations
CKD-EPI (2012) Creatinine and Cystatin C KDIGO, NKF
- more accurate than CKD-EPI based on either creatinine or cystatin C alone
PEDIATRIC eGFR EQUATIONS (18 and under)
Equation Biomarker Recommended Comments SCHWARTZ (Updated Bedside) Creatinine KDIGO, NKF, NIDDK Most commonly used equation to estimate eGFR in younger people. SCHWARTZ (Modified) Creatinine, BUN KDIGO Cystatin C-based equations Cystatin C Pediatric equations derived using cystatin C results were based on testing not yet calibrated to the reference material. Overestimation in eGFR may be observed.
CKD-EPI - Chronic Kidney Disease Epidemiology Collaboration; KDIGO - Kidney Disease Improving Global Outcomes; NIDDK - National Institute of Diabetes and Digestive and Kidney Diseases; NKF - National Kidney Foundation
- Is an eGFR always calculated when a creatinine test is ordered?
This practice is recommended and has been adopted by most laboratories. The eGFR can always be calculated at your healthcare provider's request.
- Why might my healthcare provider repeat my eGFR test?
Besides for periodic monitoring, the eGFR might be repeated if your healthcare provider feels that a temporary condition may be affecting your results.
- What other findings might suggest kidney dysfunction?
Diabetics and others at risk for developing kidney disease may be monitored for small amounts of albumin in their urine by performing a urine albumin test. The presence of albumin and other plasma proteins as well as blood in the urine can all be signs of potential kidney damage.
- Why are factors such as age, sex, and race used in eGFR calculations?
This is because these factors are known to affect the level of creatinine in the blood. Creatinine is a waste product produced by muscles from the breakdown of a compound called creatine. According to the National Kidney Foundation, men tend to have more muscle mass than women so their creatinine levels tend to be higher. African Americans have a higher average muscle mass and generate more creatinine. As we age, we lose muscle mass and thus have lower blood creatinine levels for the same kidney function the older we get.
- Is there anything else I should know?
The actual amount of creatinine that you produce and eliminate is affected by your muscle mass and by the amount of protein in your diet. Men tend to have higher creatinine levels than women and children.
A person's GFR decreases with age and some illnesses. The GFR usually increases during pregnancy.
The eGFR may be affected by a variety of drugs, such as gentamicin, cisplatin, and cefoxitin, that increase creatinine levels, and by any condition that decreases blood flow to the kidneys.
The calculation of eGFR is intended to be used when kidney function and creatinine/cystatin C production are stable. If a creatinine or cystatin C level is measured when the kidney function is changing rapidly, such as with acute kidney injury, then it will not give a useful estimate of the filtration rate.