GFR

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. It is a calculation based on the results of a blood creatinine test along with other variables such as age, sex, and race (e.g., African-American, non-African American), depending on the equation used. (For more on the equations, see the "What is being tested?" section.)

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 status of a person's 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.

Other tests that may be done at the same time 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

A creatinine test and eGFR may be ordered when a healthcare practitioner wants to evaluate a person's 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 a person has a chronic kidney disease (CKD) or a condition such as diabetes or hypertension that is associated with an increased risk of kidney damage.

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 testing, they may report calculated results for both African Americans and non-African Americans. The healthcare practitioner uses the result that applies to the particular patient 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. (Because the calculation works best for estimating reduced kidney function, actual numbers are only reported once values are less than 60 mL/min/1.73m²).

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 a person has an eGFR less than 60 mL/min/1.73m² for more than three months.

A person 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, a person's eGFR should be interpreted in relation to the person's clinical history and presenting conditions.

The following table summarizes estimated GFR and the stages of kidney damage:

Another method of evaluating kidney function and potentially estimating GFR involves the measurement of the blood level of cystatin C. There is increasing interest in the use of this test for these purposes and several studies have been performed comparing calculations of eGFR using creatinine, cystatin C, or both. According to the National Kidney Foundation (NKF), two meta-analyses concluded that cystatin C is superior to creatinine as a marker of kidney function. The NKF also states that a formula for calculating eGFR that includes both blood creatinine and cystatin C values may improve that estimate (see Common Questions #7).

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.

The actual amount of creatinine that a person produces and excretes is affected by their muscle mass and by the amount of protein in their diet. Men tend to have higher creatinine levels than women and children.

A person's GFR decreases with age and some illnesses and usually increases during pregnancy.

A slightly different equation should be used to calculate the eGFR for those under the age of 18 (see Common Questions #2). The eGFR equations are not valid for those who are 70 years of age or older because muscle mass normally decreases with age.

An eGFR may not be as useful for those who differ from normal creatinine concentrations. This may include people who have significantly more muscle (such as a body builder) or less muscle (such as a muscle-wasting disease) than the norm, those who are extremely obese, malnourished, follow a strict vegetarian diet, ingest little protein, or who take creatine dietary supplements.

The eGFR may also 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 for eGFR is intended to be used when kidney function and creatinine production are stable. If a creatinine level is measured when the kidney function is changing rapidly, such as with acute kidney failure, then it will not give a useful estimate of the filtration rate.