What is the eGFR blood test?
How is eGFR calculated?
What do eGFR results mean?
Factors that can affect eGFR accuracy
Diagnostic efficacy of eGFR blood test
The estimated glomerular filtration rate (eGFR) is a blood test to assess kidney function. The formula for calculating eGFR takes the patient’s body size, age, and sex into account to predict how well the kidneys are functioning.
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What is the eGFR blood test?
The eGFR blood test is a measure of how well the kidneys are functioning to remove waste products, such as creatinine, from the blood.1
Accurate measurement of GFR (measured GFR) is a complex and time-consuming process, making it impractical for both clinicians and patients. The estimated GFR (mGFR) is, thus, used primarily for research and transplant purposes.2
Instead, healthcare professionals use a formula to estimate GFR (eGFR). The standard method of estimating GFR includes a simple blood test that measures blood levels of creatinine, which is a muscle tissue and dietary protein breakdown product.2
Creatinine is produced at a constant rate in the body and is only removed from circulation by the kidneys. This makes creatinine a perfect agent for understanding how well kidneys filter out waste products from the body.3
The eGFR blood test is used by physicians to monitor kidney health and determine the onset of kidney diseases. Physicians often recommend this test for patients who are at higher risk of developing chronic kidney disease, such as aged patients, obese or overweight patients, patients with diabetes or hypertension, and those with a family history of chronic kidney disease or kidney failure.2
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How is eGFR calculated?
Healthcare professionals use various formulas to measure eGFR. These formulas consider the patient’s blood creatinine level, age, sex, body weight and height, and race or ethnicity to provide the result.4
Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) is the most widely used formula for eGFR in the UK. This formula considers multiple factors that can influence creatinine production, such as age, sex, and race or ethnicity.3
However, this formula is not perfectly accurate. It often provides false results in individuals whose muscle mass significantly differs from that commonly observed in their peers of the same age, sex, and ethnic origin. 3
The Modification of Diet in Renal Disease (MDRD) Study formula is also used to calculate eGFR. This formulation, which was developed in individuals with chronic kidney disease, has some major limitations, including imprecision and systematic underestimation of measured GFR at higher levels. 5
Cystatin C blood test is a newer method for estimating eGFR. Although not widely available yet, this test is more accurate than the creatinine blood test in determining kidney functions. Like creatinine, cystatin C is produced at a constant rate and is only removed from the circulation through kidney filtration. 3
The eGFR is reported in milliliters per minute per 1.73m2, i.e., milliliters of cleansed blood per minute per body surface. Its value may vary depending on age, sex, and other factors and naturally declines with increasing age and muscle mass loss. 4
The average eGFR for people aged 20 – 29 years is about 116 mL/min/1.73m2, which reduces to 85 mL/min/1.73m2 for people aged 60 years or above. The normal eGFR value is 90 or above. However, a value as low as 60 may also indicate normal kidney function if there are no other kidney-related symptoms. 3
What is eGFR, and how is it used to measure kidney function?
What do eGFR results mean?
Physicians recommend an eGFR blood test to evaluate whether a patient has chronic kidney disease or not. An eGFR value of less than 60 for three consecutive months is indicative of chronic kidney disease. 4
There are five stages of kidney disease. Stage 1, defined by an eGFR value of 90 or above, indicates normal kidney functions with mild signs of kidney damage, such as physical kidney damage or protein in urine. 1
Stage 2, defined by eGFR values between 60 and 89, indicates mild loss of kidney functions. Stage 3, defined by eGFR values between 30 and 59, indicates moderate loss of kidney functions. At this stage, patients may experience some symptoms of kidney damage, such as hand and foot swelling, back pain, and more frequent or less frequent urination. 1
Stage 4, defined by eGFR values between 15 and 29, indicates poor kidney function, with moderate to severe kidney damage. An eGFR value of less than 15 refers to stage 5, which is a sign of kidney failure. This stage is often life-threatening and indicative of dialysis or kidney transplant. 4
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Factors that can affect eGFR accuracy
Although creatinine is the most widely used marker for kidney function assessment, several factors can potentially influence blood levels of creatinine. Creatinine-based eGFR test performs better in individuals with a stable kidney function. 6
The eGFR blood test may not provide accurate results in patients with unstable creatinine concentrations, including pregnant women, hospitalized patients with acute kidney injury, cancer patients, critically ill patients, patients with neuromuscular diseases, or those with serious comorbidities. 6
Vegetarian or low-meat diets, as well as diets with poor nutritional quality, can also affect the accuracy of creatinine-based eGFR test results. An extreme body size and muscle mass, certain medications, body hydration status, and some endogenous substances (elevated glucose, ketone bodies, and bilirubin) can affect eGFR blood test results. 6
Diagnostic efficacy of eGFR blood test
Monitoring eGFR is vital for patients who are at higher risk for developing kidney disease or who already have kidney disease. Frequent urination, bloody or foamy urine, dry, itchy skin, extreme fatigue, muscle cramps, nausea or vomiting, shortness of breath, and hand and foot swelling are some of the signs that may indicate kidney disease. 4
An abnormal eGFR is an early indication of kidney disorders. Physicians may recommend other tests for patients with abnormal eGFR to assess kidney damage or identify its causes.
The most commonly recommended tests are urinalysis for albumin, kidney ultrasound or CT scan for kidney stones, cancer, or urinary tract problems, creatine clearance test for simultaneous measurement of blood and urine creatinine levels, and kidney biopsy for determining the type and severity of kidney disease. 4
References
1. Blood test: eGFR (estimated glomerular filtration rate) | American Kidney Fund. November 10, 2021. Accessed February 22, 2025. https://www.kidneyfund.org/all-about-kidneys/tests/blood-test-egfr
2. Estimated Glomerular Filtration Rate (eGFR) | National Kidney Foundation. Accessed February 22, 2025. https://www.kidney.org/kidney-topics/estimated-glomerular-filtration-rate-egfr
3. Understanding test results. Kidney Research UK. Accessed February 26, 2025. https://www.kidneyresearchuk.org/kidney-health-information/living-with-kidney-disease/how-can-i-help-myself/understanding-test-results/
4. Estimated Glomerular Filtration Rate (eGFR): Definition & Results. Cleveland Clinic. Accessed February 22, 2025. https://my.clevelandclinic.org/health/diagnostics/21593-estimated-glomerular-filtration-rate-egfr
5. Levey AS, Stevens LA, Schmid CH, et al. A New Equation to Estimate Glomerular Filtration Rate. Ann Intern Med. 2009;150(9):604-612.
6. Clinical Measurements & eGFR Accuracy - NIDDK. National Institute of Diabetes and Digestive and Kidney Diseases. Accessed February 26, 2025. https://www.niddk.nih.gov/research-funding/research-programs/kidney-clinical-research-epidemiology/laboratory/factors-affecting-egfr-accuracy/clinical-measurements
Further Reading