Glomerular Filtration Rate (GFR) Test
A GFR test is a blood test that estimates how well your kidneys are filtering waste from your blood, serving as a key indicator of kidney health.
What is a GFR Test?
Your kidneys contain tiny filters called glomeruli that remove waste and extra water from your blood. A GFR test measures how much blood these filters process each minute. Because direct measurement is complex, providers typically use an eGFR (estimated GFR), which is calculated using a formula based on your age, sex, height, weight, and levels of creatinine—a waste product from muscle activity—or cystatin C in your blood.
Why Do I Need a GFR Test?
Because early-stage kidney disease often lacks symptoms, the test is used for routine screening. You may be at higher risk and require testing if you have:
- Diabetes or high blood pressure
- A family history of kidney disease
- Heart disease
- Obesity or are a smoker
- Reached age 50 or older
If kidney disease progresses, symptoms may include swelling in the limbs or face, changes in urination frequency, fatigue, muscle cramps, or nausea.
What Happens During the Test
A health care professional draws a blood sample from a vein in your arm. You may need to fast or avoid certain foods before the test, and you should inform your provider of all medications you are taking.
Understanding Results
Results are reported as a number indicating how much blood your kidneys filter per minute.
- Normal eGFR: Suggests healthy kidney function.
- Below normal/low eGFR: May indicate kidney disease.
- Very low eGFR: May indicate kidney failure.
Providers may also order additional tests, such as a protein in urine test or a BUN (blood urea nitrogen) test, to further evaluate kidney health.