Estimated Glomerular Filtration Rate (eGFR)

A test of glomerular filtration rate (GFR) evaluates kidney function. The kidney consists of tiny filters which are called glomeruli that remove waste material from the blood. The GFR test is an assessment of how well this filtration process is working.

There are several methods to test GFR. Most often the glomerular filtration rate is estimated by measuring another substance. Many estimated glomerular filtration rate (eGFR) tests use a formula based on the levels of creatinine, a waste product produced by the body’s muscles, in the blood.

Tests of eGFR are commonly used to detect kidney failure or other kidney problems and may also be utilized before beginning medical procedures or treatments that can affect the kidney.

Purpose of the test

The purpose of a glomerular filtration test is to determine if the kidney is functioning properly. As one of the principal evaluations of kidney health, this test has many potential applications. For example, it can be used for diagnosis, screening, and monitoring of kidney problems.

Diagnosis

Diagnosis involves testing that happens after symptoms have occurred. Diagnosis tries to determine the nature of the problem that is causing those symptoms. An eGFR test can be part of the diagnostic process for symptoms that could be tied to a kidney problem such as urinary changes, fatigue, swelling in the arms or legs, itching, nausea, and vomiting.

In addition to detecting kidney problems, an eGFR test may be used to assess the severity or stage of kidney disease, including the possible presence of kidney failure. However, changes in eGFR do not perfectly correspond to kidney health, so other tests are often needed to evaluate the degree of damage to the kidney.

Screening

Screening is the medical term for testing that looks for signs of a health problem before any symptoms have occurred. An eGFR test may be used for detection of kidney disease in people with risk factors such as diabetes, heart disease, urinary blockages or infections, a family history of kidney issues, high blood pressure, and chronic kidney disease.

Screening with an eGFR test may also be done for people who are going to donate a kidney. The test can help make sure they have sufficient kidney function to safely donate and can screen for problems after donation.

Doctors may prescribe an eGFR test as a form of screening before some medical procedures such as imaging studies with contrast agents or treatments that can affect the kidney. In this way, the eGFR test is used to try to prevent complications or side effects from medical care.

Monitoring

Monitoring is part of ongoing care that observes how kidney function changes over time or in response to treatment. If you have been diagnosed with kidney disease, repeat tests of your glomerular filtration rate may be one component of how your doctor assesses the condition of your kidneys.

An eGFR test can also be used to monitor kidney function if you are taking a medication that has the potential to damage the kidneys.

What does the test measure?

The test provides a calculation of the glomerular filtration rate, which reflects how well the kidneys are filtering the blood and is represented in milliliters per minute (mL/min). The result is often listed as milliliters per minute per 1.73 square meters of body surface area (mL/min/1/.73m2).

In an eGFR test, this rate is not measured directly. Instead, it is estimated by measuring another substance in the blood. Most often, creatinine is measured, and then special formulas calculate eGFR based on the level of creatinine in the blood.

Creatinine is naturally produced as a result of the body’s process of supplying energy to muscles. The kidneys filter and remove creatinine from the blood. Typically the amount of creatinine generated is relatively constant, so changes in creatinine levels in the blood can be an indicator of abnormal kidney function.

Not everyone produces the same amount of creatinine, though, which is why eGFR calculators can include factors such as age, sex, height, and ethnicity along with your creatinine levels in order to estimate the glomerular filtration rate.

While commonly used, creatinine is not the only substance that can be used to estimate GFR. For example, some eGFR tests measure cystatin C, which is a protein made by many types of cells in the body. The effects of individual factors, such as muscle mass and diet, are different when calculating eGFR using cystatin C, so this method of eGFR testing may be more accurate in some circumstances.

The underlying measurement and formula used to estimate GFR may be detailed on an eGFR test report. Your doctor can also provide this information about your test.

When should I get an eGFR test?

There are a number of circumstances in which an eGFR test may be appropriate and beneficial for your medical care.

An eGFR test can help identify serious kidney disease and kidney failure. In response to a wide range of symptoms, an eGFR test can help determine if your kidneys are working properly.

Sometimes kidney disease does not cause immediate symptoms. In these cases, testing with eGFR may enable an earlier diagnosis and improved ability to slow the progression of the disease. In general, this type of screening with an eGFR test is only done in people who have an elevated risk of kidney problems.

When you have previously been diagnosed with kidney disease or have had abnormal eGFR tests, doing one or more repeat tests may aid in the diagnosis and monitoring of your condition.

In some cases, an eGFR test is conducted before kidney donation or the initiation of certain medical treatments or procedures. In some of these circumstances, though, a more precise test may be necessary to directly measure rather than estimate GFR.

Your doctor is in the best position to address whether an eGFR test or any other test of kidney function is appropriate for you given your overall health and health history.

How to get tested

An estimated glomerular filtration rate test is usually done after being ordered by a doctor, and the blood draw generally takes place at a medical site like a doctor’s office or hospital.

Can I take the test at home?

Although most kidney function testing is prescribed by a doctor and carried out in a medical facility, some at-home options exist for eGFR testing.

With at-home kits, only the sample collection occurs at home. Once a sample is taken, it must be sent to a laboratory where it can be analyzed, and results are made available electronically.

At-home tests generally do not provide direct medical consultation and interpretation of your results, so the test report will normally need to be discussed with your doctor.

How much does the test cost?

The price for an eGFR test can vary based on a number of factors, including:

• Where the test takes place
• The method used to calculate GFR
• Whether any other tests or measurements were performed on your blood sample
• Whether you have health insurance coverage

Costs associated with an eGFR test can include office visits, technician fees for the blood draw, and laboratory charges for analyzing your blood. Health insurance often pays for some or all of these costs when the test is prescribed by a doctor, but you should check with your insurance provider to find out about any out-of-pocket costs for your deductible or co-payments.

At-home tests of eGFR are often available for around $100, which includes the costs of shipping your blood sample to the lab.

An estimated glomerular filtration rate test is a type of blood test. The blood sample for the test is taken with a needle that is inserted into a vein in your arm. This routine procedure is usually done at a doctor’s office, hospital, or laboratory.

For an at-home test, a drop of blood must be taken from your fingertip and applied to a test strip that can be sent to a medical laboratory.

Before the test

Taking a blood sample for an eGFR test is a straightforward procedure, but some preparation may be needed before the test:

• Fasting: In some cases, you will be asked to not eat or drink any liquids other than water for a number of hours before the test.
• Medication review: Several types of medications can affect the ability to accurately calculate your eGFR. For this reason, you should talk with your doctor about your active prescriptions as well as any over-the-counter drugs or dietary supplements that you are taking.

When scheduling your test, you can ask your doctor’s office about whether you need to fast or adjust your medications beforehand.

Pregnancy can also influence an eGFR test, so make sure to tell your physician if you are pregnant or think you might be pregnant.

For at-home test kits, it is essential to read the instructions in full before taking the test. The instructions will explain any necessary preparations and the procedure for properly obtaining and shipping your blood sample.

During the test

A blood draw for an eGFR test is a routine outpatient procedure. A nurse or phlebotomist will start by placing an elastic band on the upper part of your arm so that more blood flows through the veins in that arm. They will clean the skin near where the blood will be drawn and then will insert a needle into the vein to withdraw a vial of blood.

Some people experience pain during the blood draw, including a possible sting when the needle is inserted and removed. In total, the procedure is usually over within a few minutes.

For an at-home test, the blood sample comes from your fingertip. The test kit includes a tiny needle to prick your finger, and then your blood must be applied to a test strip. Instructions are provided for how to prepare the test strip for being mailed to the lab.

After the test

A blood draw is normally a quick procedure. After the sample is taken, a bandage or cotton swab will be placed over the puncture site to stop the bleeding.

You may notice some tenderness or bruising in your arm. These and other side effects usually go away quickly. Talk to your doctor if you notice any severe or persistent side effects or any signs of infection.

If you are required to fast for the test, you may want to bring a snack to eat once your blood is drawn. You should be able to drive and do most normal activities after the test.

After a fingerstick for an at-home collection kit, you may need to use a bandage if your finger continues to bleed. Other side effects of a fingerstick test are uncommon.

Receiving test results

The results from an estimated glomerular filtration rate test are usually available within several business days after your blood sample is taken. The results may be provided by your doctor, sent to you by mail, and/or made available online through a secure health portal.

At-home tests can take a few days longer to get results because of the need to ship the test strip to the lab. When the sample has been analyzed, results are accessible through a website or smartphone app.

Interpreting test results

When results are available, you should receive a test report that lists your estimated glomerular filtration rate. Your eGFR result should be displayed in milliliters per minute per body surface area. A surface area of 1.73 square meters is used in many calculations but can be adjusted for individual patients.

In addition to eGFR, the test report usually lists an additional test, most often creatinine, that was used to calculate eGFR.

A reference range for what the laboratory considers to be normal eGFR levels is typically included with your test result. For eGFR based on creatinine levels, many laboratories list separate reference ranges for African Americans and non-African Americans because some prior studies have found varying eGFR levels among different racial and ethnic groups. Recently, though, organizations focused on kidney health have explored ending this way of reporting eGFR reference ranges for different ethnic groups.

Interpretation of an eGFR test is focused on determining if you may have reduced kidney function or kidney disease. When eGFR is very low, generally under 15 mL/min/1.73m2, it can be a sign of kidney failure or serious disease. An eGFR level that is below 60 mL/min/1.73m2 for 3 months is considered to be an indication of chronic kidney disease.

Although an eGFR test can identify kidney problems, injuries to the kidney are not perfectly reflected in a lower eGFR. This is because some kidney cells can increase filtration to compensate for kidney damage.

In addition, a healthy or normal eGFR level can vary based on individual factors. For example, a normal eGFR is usually lower in older adults, females, and people with a smaller body size. Drugs and diet can also affect creatinine and calculations of eGFR.

Your doctor may consider these factors as well as your overall health and other test results when interpreting your eGFR test result. Your doctor can best explain your test result, including whether your eGFR is normal or abnormal, and what that result means for your health.

Are test results accurate?

An eGFR test is a straightforward and generally accurate method of assessing kidney function. In the United States, efforts to calibrate and standardize creatinine measurements have improved the dependability of using creatinine levels to estimate GFR.

However, eGFR is not a perfect test, and several factors can affect its accuracy. For example, eGFR calculations based on creatinine levels are only accurate when a person has stable kidney function. The test is generally not useful or recommended in patients with acute illness, including many who are hospitalized or have serious co-existing conditions.

In addition, most eGFR tests are based on creatinine levels, and test accuracy is affected by individual factors that can influence creatinine levels and the calculation of eGFR. Some of these factors include:

• Age: eGFR tends to decrease with age as muscle mass decreases. Only a few studies have been conducted to validate formulas for estimating GFR in older adults.
• Body size: Creatinine is a product of normal muscle activity, so individual differences in body size and muscle mass can impact creatinine levels and eGFR calculations. Examples of patients whose eGFR accuracy may be affected include people with obesity, amputees, bodybuilders, or people with disorders causing malnutrition or muscle loss. Calculations of eGFR can be modified for body size, but there may be variability in results based on how these adjustments are made and interpreted.
• Sex: A normal eGFR is generally considered to be lower in females, although this can be affected by other individual factors like muscle mass.
• Race and ethnicity: Historically, a patient’s race and ethnicity has been taken into account when calculating eGFR. Recently, though, these adjustments have been called into question, highlighting a need for more tailored, accurate, and inclusive methods of considering race and ethnicity in eGFR tests.
• Diet: Creatinine levels can fluctuate in response to diet. For example, creatinine levels may change by up to 30% after eating red meat. Some dietary supplements can also affect creatinine levels.
• Pregnancy: eGFR typically increases during pregnancy, affecting the applicability of the test for people who are pregnant.
• Medications: Many different prescription and over-the-counter drugs can provoke changes in eGFR measurements.

To reduce the effects of these individual factors, eGFR may be estimated by measuring a substance called cystatin C instead of creatinine. In some cases, two eGFR calculations—one using creatinine and one using cystatin C or a combination of both—may be performed. This serves as a type of initial and confirmatory testing that may improve overall accuracy of estimating GFR.

Do I need follow-up tests?

Follow-up tests are common if an eGFR test is abnormal or inconclusive. A repeat test may be used to observe trends in glomerular filtration rate. In addition, in some circumstances, the doctor may recommend using a different method to estimate eGFR, such as with cystatin C. A direct measurement of GFR may be considered when greater test precision is required.

Although eGFR testing can detect abnormal kidney function, it cannot pinpoint the underlying cause. As a result, other tests, including blood or urine tests, imaging tests, or a kidney biopsy, may be appropriate follow-up examinations.

Depending on your test results, symptoms, and health history, you may be referred to a doctor that specializes in kidney issues, known as a nephrologist, who can coordinate your follow-up testing and care.

Questions for your doctor about test results

Your doctor can help you understand the significance of an eGFR test for your overall health. Some questions that can help you discuss your results with your doctor include:

• What was my eGFR test result? Is that considered normal or abnormal?
• What does this test result indicate about my kidney function?
• How was my GFR estimated? Is that estimation method accurate in my situation?
• Is any other test warranted to calculate eGFR again or to directly measure GFR?
• Do you suggest any follow-up tests?

Several kinds of tests can be used to evaluate kidney function, and the following sections compare and contrast eGFR and some of these related tests.

How is estimating GFR different from measuring GFR?

There are different ways to assess the glomerular filtration rate, and estimating GFR is distinct from directly measuring GFR.

Measuring GFR is possible by infusing a substance, such as a compound called inulin or certain radioisotopes, into the blood and seeing how quickly it is cleared by the kidneys. While more accurate, this type of testing is expensive and requires more specialization, reducing its ability to serve as a widespread method of assessing kidney function.

Accordingly, direct GFR measurement is reserved for particular situations, and tests that estimate GFR based on creatinine or cystatin C are much more frequently used.

How is an eGFR test different from a creatinine test?

While many eGFR tests estimate GFR using a single creatinine measurement and a complex calculation, eGFR and creatinine are not the same.

Creatinine is a byproduct of normal muscle activity. The kidneys filter creatinine out of the blood so that it can be cleared from the body. If the kidneys are not working properly, though, creatinine levels can rise.

In this way, GFR can be estimated using creatinine levels, but the two do not have a perfectly proportional relationship.

How is an eGFR test different from a cystatin C test?

Cystatin C is a substance found in the blood that is produced by many cells in the body. It is distinct from GFR but, like creatinine, can be used to estimate GFR.

How is an eGFR test different from a creatinine clearance test?

Creatinine is found in both the blood and the urine. In a creatinine clearance test, the levels of creatinine in blood and urine are compared, and this can inform calculations of GFR.

Conducting a creatinine clearance test requires collecting all urine produced over a 24-hour period, which may be inconvenient. Under- or over-collection of urine during this time can significantly alter test results.

Using creatinine clearance for eGFR also tends to overestimate actual GFR and may require additional adjustments in the calculations or in the interpretation of test results.

How is an eGFR test different from a basic or comprehensive metabolic panel test?

Both the basic metabolic panel (BMP) and comprehensive metabolic panel (CMP) are blood tests that help to assess how well the kidneys are working; however, the standard version of these tests do not include a calculation of eGFR.

That said, the BMP and CMP involve a measurement of creatinine, so these tests may enable the lab or your doctor to estimate GFR.

In addition, the BMP and CMP include assessments of more than just kidney function. For example, these tests measure blood sugar and levels of electrolytes that facilitate many important processes in the body.

Other related tests

• Creatinine
• Creatinine Clearance
• Cystatin C
• Urine Albumin and Albumin to Creatinine Ratio (ACR)
• Albumin Blood Test
• Urine Protein and Urine Protein to Creatinine Ratio

For more information about how the kidneys work and conditions that can affect them, you can review the following links:

• National Library of Medicine: Kidneys and Urinary System
• National Institute of Diabetes and Digestive and Kidney Diseases: Kidney Disease
• National Library of Medicine: Kidney Diseases

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