eGFR stands for estimated glomerular filtration rate. It is a number calculated from a blood test—usually creatinine—that helps show how well your kidneys filter waste and extra fluid from your blood. A higher eGFR generally means better filtering, while a lower result may need follow-up. One result cannot diagnose kidney disease by itself; clinicians consider trends over time and other tests, including urine albumin.
What Does eGFR Measure?
Your kidneys filter your blood through millions of tiny structures called glomeruli. These filters help remove waste products and excess fluid, which are then eliminated from your body through urine. The glomerular filtration rate, or GFR, describes how much blood the kidneys filter over a given period.
Because directly measuring GFR requires specialized testing, healthcare professionals commonly use an estimated value called eGFR. According to MedlinePlus, this estimate is usually calculated from the level of creatinine in the blood, along with information used in a mathematical formula. The result provides a practical way to assess kidney filtering function without requiring a specialized measurement procedure.
eGFR is generally reported in milliliters per minute per 1.73 square meters of body surface area (mL/min/1.73 m²). This standardizes the estimate to a reference body size so results can be compared more consistently. The figure is an estimate rather than a direct measurement of the exact amount of blood your kidneys filter.
How Is eGFR Calculated?
The calculation usually begins with a blood creatinine test. Creatinine is a waste product produced during normal muscle activity. Healthy kidneys remove creatinine from the blood, so changes in its level can help provide information about kidney function.
The calculation also accounts for factors such as age and sex, depending on the equation used. Some estimates use cystatin C, another substance measured in the blood. In certain situations, combining creatinine and cystatin C can provide a more accurate estimate than relying on creatinine alone. The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) explains that current adult equations can use creatinine, cystatin C, or both to estimate kidney filtration.
A direct measurement of GFR is also possible, but it involves specialized testing and is not routinely needed for everyone. A healthcare professional may consider it when a more precise measurement is important or when medical circumstances make the usual estimate less reliable.
Why Do Doctors Order an eGFR Test?
An eGFR test helps healthcare professionals screen for kidney problems, investigate possible kidney disease, and monitor kidney function over time. It is often included in routine blood testing, particularly when someone has a condition or medical history that increases the likelihood of kidney disease.
According to the NIDDK's guidance on kidney disease testing, healthcare professionals commonly assess kidney health using both a blood test for GFR and a urine test for albumin. This is important because kidney disease may develop without noticeable symptoms in its earlier stages.
An eGFR test may be considered for people with diabetes, high blood pressure, heart disease, or a family history of kidney failure. It can also help monitor people who already have kidney disease or whose treatment requires closer attention to kidney function. The appropriate testing schedule depends on a person's medical history, existing results, and clinical needs.
How Should You Understand an eGFR Result?
In general, a higher eGFR suggests that the kidneys are filtering blood more effectively, while a lower-than-expected result may indicate reduced kidney function. However, the number must be interpreted in context. Age, other health conditions, the person's previous results, and additional evidence of kidney damage can all influence what the result means.
For many adults, an eGFR of 90 or higher can be consistent with usual kidney filtering, but this value does not rule out kidney damage. An eGFR below 60 may indicate reduced kidney function. When an eGFR below 60 persists for at least three months, it meets one important criterion used to identify chronic kidney disease. Laboratory methods and reference information may vary, so a healthcare professional should interpret the result rather than relying on a number alone.
The National Kidney Foundation explains that chronic kidney disease can be identified by kidney abnormalities that persist for more than three months. Reduced eGFR is one possible finding, but other evidence, such as persistent albumin in the urine or structural changes in the kidneys, can also be relevant. A single result does not establish how long a problem has been present or what caused it.
It is also possible to have evidence of kidney damage even when eGFR is relatively well preserved. For this reason, clinicians assess the overall pattern rather than treating eGFR as a complete picture of kidney health.
What Can Affect an eGFR Result?
Although eGFR is useful, it is not a perfect measure. Its accuracy depends partly on how closely the assumptions in the calculation match the person being tested. Creatinine-based estimates can be affected by factors unrelated to kidney filtering, which may make the result appear higher or lower than the person's actual filtration rate.
- Muscle mass: People with unusually high or low muscle mass may produce different amounts of creatinine, affecting the estimate.
- Diet: Eating meat shortly before testing can influence creatinine levels in some circumstances.
- Medicines and other health conditions: Certain medicines or medical conditions may alter creatinine levels or make an estimate less reliable.
- Age and pregnancy: Age affects the interpretation of kidney function, while pregnancy can change how kidney filtration is assessed.
- Changes over time: A result that differs from earlier tests may need to be assessed alongside the timing of the tests and any recent illness or other changes in health.
The National Kidney Foundation's eGFR guidance describes several factors that can affect the accuracy of an estimate, including muscle size, diet, pregnancy, and certain medical conditions. If these factors are relevant, a clinician may use cystatin C testing, repeat blood tests, or another method to obtain a clearer assessment.
Why Are Repeat Tests and Urine Tests Important?
A single eGFR result is a snapshot of kidney function at one point in time. Comparing results over time helps healthcare professionals determine whether kidney function appears stable or is changing. Depending on the finding and the person's circumstances, they may recommend repeating the blood test to confirm whether an unexpected result persists.
A urine albumin-to-creatinine ratio (uACR) is another important test. It measures the amount of albumin, a protein in the blood, relative to creatinine in a urine sample. Healthy kidneys generally keep most albumin in the bloodstream. Albumin appearing in the urine can be a sign of kidney damage, even when eGFR does not appear substantially reduced.
Depending on the circumstances, a clinician may also review blood creatinine, other blood tests, urine findings, blood pressure, medical history, and imaging results. These pieces of information help explain whether an eGFR finding needs further investigation and whether additional monitoring is appropriate.
What Happens During an eGFR Test?
An eGFR test usually requires a blood sample taken from a vein in your arm. The laboratory measures creatinine and calculates the estimated filtration rate using an appropriate formula. The blood draw generally takes only a few minutes, and temporary discomfort or minor bruising may occur at the needle site.
Preparation depends on the laboratory and the tests ordered alongside eGFR. You may be asked to avoid certain foods or fast for a specified period. Tell your healthcare professional about prescription medicines, over-the-counter medicines, vitamins, and supplements you take because some can affect the results. Do not stop a prescribed medicine unless your healthcare professional tells you to.
What Should You Do If Your eGFR Is Lower Than Expected?
Discuss the result with the healthcare professional who ordered the test. Ask how it compares with your earlier results, whether it should be repeated, and whether a urine albumin test or cystatin C measurement would provide additional information. The next step depends on the result, your medical history, and any other findings.
Do not change prescribed medicines or start a kidney-related supplement solely because of an eGFR result. Some medicines require adjustments when kidney function changes, but the appropriate decision depends on the medicine and the clinical situation. A healthcare professional can explain whether any changes or further assessment are needed.
People with diabetes, high blood pressure, or other risk factors for kidney disease should ask their healthcare professional how often kidney function needs to be checked. Regular monitoring can help identify changes early, including changes that occur before noticeable symptoms develop.