About the Test
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 diagnose, screen, and monitor kidney problems.
This involves testing after signs and/or symptoms have occurred. It tries to determine the nature of the problem 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.
This is a medical term for testing that looks for signs of a health problem before any symptoms have occurred. An eGFR test may be used to detect kidney disease if you have 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 if you are going to donate a kidney. The test can help ensure you have sufficient kidney function to donate safely and 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 prevent medical care complications or side effects.
This part of ongoing care observes how kidney function changes over time or in response to treatment. If you have been diagnosed with kidney disease, repeat tests of your GFR 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?
It calculates the GFR, which reflects how well the kidneys filter 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 by the body 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, which is why eGFR calculators can include factors such as age, sex, and race along with your creatinine levels to estimate the GFR.
While most commonly used, creatinine is not the only substance that can be used to estimate GFR. For example, some eGFR tests measure cystatin C, a protein made by most types of cells in the body. Unlike creatinine, the blood level of cystatin C is less influenced by differences in our body, such as muscle mass, physical activity, and diet.
Therefore cystatin C-based calculation of eGFR is more accurate in some circumstances. You may ask your doctor if cystatin C-based eGFR is a better test for you.
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 this test?
There are many 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 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 if you have an elevated risk of kidney problems.
When you have previously been diagnosed with kidney disease or had abnormal eGFR tests, doing one or more repeat tests may aid in diagnosing and monitoring your condition.
In some cases, an eGFR test is conducted before kidney donation or the initiation of certain medical treatments or procedures. Some of these circumstances may require a more precise test to directly measure rather than estimate GFR.
Your doctor is in the best position to address whether an eGFR test or any other kidney function test is appropriate, given your overall health and health history.
Finding an eGFR Test
How can I get an eGFR test?
An eGFR test is usually done after being ordered by a doctor. 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 many factors, including:
- Where the test takes place
- The method used to calculate GFR (for example, creatinine versus cystatin C)
- 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 check with your insurance provider if there are 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.
Taking an eGFR Test
An eGFR test is a type of blood test. The blood sample for the test is taken with a needle 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. Still, some preparation may be needed before the test:
- Fasting: In some cases, you will be asked not to eat or drink any liquids other than water for several hours before the test.
- Medication review: Several medications can affect the ability to accurately calculate your eGFR. For this reason, talk with your doctor about your active prescriptions and any over-the-counter drugs or dietary supplements you are taking.
When scheduling your test, you can ask your doctor’s office whether you need to fast or adjust your medications beforehand.
Pregnancy can also influence an eGFR test, so tell your physician if you are pregnant or think you might be.
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 procedures for properly obtaining and shipping your blood sample.
During the test
A blood draw for an eGFR test is a routine outpatient procedure. The nurse or phlebotomist will place 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 insert a needle into the vein to withdraw a vial of blood.
You may 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 your blood must be applied to a test strip. Instructions are provided for preparing 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 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 a bandage if your finger continues to bleed. Other side effects of a fingerstick test are uncommon.
eGFR Test Results
Receiving test results
The results from an eGFR test are usually available within several business days after your blood sample is taken. 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 eGFR. Your 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 used to calculate eGFR.
Your test result typically includes a reference range for what the laboratory considers normal eGFR levels. 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 groups.
Recently, though, organizations focused on kidney health have recommended ending this way of reporting eGFR reference ranges for different racial groups. Almost all clinical laboratories have already implemented race-neutral eGFR calculation.
An eGFR test is interpreted to determine if you may have reduced kidney function or kidney disease. When eGFR is very low, generally under 15 mL/min/1.73m2, it can signify kidney failure or serious disease. An eGFR level below 60 mL/min/1.73m2 for three months is considered 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. They can best explain your test result, including whether your eGFR is normal or abnormal and what that result means for your health.
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?