Glomerular Filtration Rate (GFR) Test: What It Is, Why It Is Done, and How to Get Ready

What is this test? The glomerular filtration rate (GFR) test measures how well your kidneys filter waste products and extra fluid out of your blood. It is considered one of the best overall indicators of kidney health and is used to detect and track chronic kidney disease.

The Adult GFR (Glomerular Filtration Rate) test measures how well your kidneys are working to filter waste products from your blood. The glomeruli are tiny filters in your kidneys that normally filter about 120-125 quarts of blood daily, producing about 1-2 quarts of urine. GFR tells your healthcare provider how much blood your kidneys are filtering per minute.

Why do I need this test? Your doctor may order this test if you have conditions that can damage the kidneys, such as diabetes, high blood pressure, or heart disease, or if you have symptoms like swelling in your legs, foamy urine, or unusual fatigue. It is also used to monitor known kidney problems and to make sure certain medicines are safe for your kidneys.

Your doctor may order a GFR test to: check how well your kidneys are working, screen for kidney disease especially if you have diabetes, high blood pressure, or a family history of kidney problems, monitor existing kidney disease, or check if kidney function is affected by certain medications.

How is the GFR calculated? GFR is usually estimated (often reported as eGFR) using a formula that combines your blood creatinine level with your age, sex, and other factors. Creatinine is a waste product that healthy kidneys remove from your blood. A higher estimated GFR generally means better kidney function, while lower numbers suggest reduced kidney function.

GFR is not directly measured but is estimated (eGFR) using a formula. The calculation uses your blood creatinine level (a waste product), your age, sex, and sometimes your race. Sometimes other factors like blood urea nitrogen (BUN) and albumin levels may also be considered.

What happens during the test? In most cases, GFR is estimated from a simple blood draw, usually taken from a vein in your arm, and the sample is sent to a laboratory for analysis. Sometimes a urine sample collected over 24 hours is also needed. The procedure is quick, and you can go home right afterward.

A healthcare professional will take a blood sample from a vein in your arm using a small needle. You may feel a brief stinging sensation. The blood sample is then sent to a laboratory for analysis. The test is simple and takes only a few minutes.

How do I prepare for the test? Little preparation is usually needed, but follow any specific instructions from your healthcare provider. You may be asked to avoid eating or drinking for several hours before the blood draw and to avoid vigorous exercise the day before. Tell your provider about all medicines and supplements you take, since some can affect creatinine levels and your results.

No special preparation is usually needed for a GFR test. However, your doctor may ask you to avoid eating large amounts of meat before the test, as this can affect creatinine levels. Always follow any specific instructions your healthcare provider gives you.

Understanding your results: A GFR of 60 mL/min/1.73m2 or higher is generally considered normal for most adults, while a result below 60 that stays low for three months or longer may indicate chronic kidney disease. Your doctor will interpret your number along with your symptoms, medical history, and other test findings, so be sure to ask what your result means for your health.

GFR results are reported as milliliters per minute (mL/min). A normal GFR for healthy adults is typically above 90 mL/min. As kidney function decreases, GFR values drop. Your doctor will interpret your results based on your overall health and other factors.