What Is GFR? Everything You Need to Know About Kidney Function

What Is GFR?

GFR (Glomerular Filtration Rate) is the specific measurement used to see how well your kidneys are filtering waste out of your bloodstream. It is widely considered the most important number for checking your overall kidney health.

Inside your kidneys, about a million tiny filters called glomeruli work constantly. Every minute, they process around 120-130 mL of blood, keeping what your body needs and getting rid of the rest. The GFR is simply the measurement of how fast that process is happening.

You can think of GFR like a speed test for your kidneys; it's a way to tell how much blood they are successfully cleaning each minute.

GFR vs. eGFR — What's the Difference?

Measuring actual GFR is a complex process that involves an injection and a 24-hour urine collection. Because it's expensive and difficult for patients, doctors usually use eGFR (estimated GFR) instead. It gives a very accurate estimate based on a simple blood test.

The eGFR formula uses:

• Serum creatinine — a waste product from muscle metabolism
• Age — kidney function naturally decreases with age
• Sex — biological sex affects creatinine levels

The current standard is the CKD-EPI 2021 equation, which is race-free. It was adopted by the National Kidney Foundation and American Society of Nephrology in 2021.

What Is a Normal GFR?

A normal eGFR is 90 or above. However, an eGFR between 60-89 can also be normal for older adults if there are no other signs of kidney damage (like protein in urine).

Why Does GFR Matter?

GFR is critical because your kidneys can lose as much as 50% of their function before you start noticing symptoms. Often, by the time you feel sick, most of the damage is already done. Regular eGFR testing is simply the most reliable way to catch problems before they get serious.

Doctors use eGFR to:

• Screen for kidney disease, especially for those with diabetes or high blood pressure
• Determine your CKD stage to help guide your treatment plan
• Adjust medication doses, as many drugs need lower doses if kidney function is reduced
• Track your health over time, since trends are usually more important than a single result

What Can Affect Your GFR?

Several factors can temporarily change your eGFR:

• Dehydration, which can cause a temporary dip in eGFR
• Heavy exercise, as intense workouts can raise creatinine levels
• Recent diet, specifically very high-protein meals right before a test
• Medications, such as ACE inhibitors, which can cause a minor, expected decrease
• Natural aging, as eGFR slowly declines as we get older (usually about 1 mL/min per year after age 30)

For this reason, CKD is typically diagnosed only after two eGFR readings below 60 that are at least 3 months apart.

How to Protect and Improve Your GFR

1. Control blood pressure — high BP is the #1 cause of kidney damage (target: <130/80)
2. Manage blood sugar — diabetes is the #2 cause of CKD
3. Stay hydrated — 6-8 cups of water daily for most people
4. Avoid NSAIDs — ibuprofen and naproxen reduce kidney blood flow
5. Don't smoke — smoking accelerates kidney function decline
6. Eat a balanced diet — moderate protein, low sodium
7. Exercise regularly — 150 minutes/week of moderate activity

When Should You Get Your GFR Tested?

Ask your doctor for an eGFR test if you have any of these risk factors:

• Diabetes (type 1 or type 2)
• High blood pressure
• Family history of kidney disease
• Age 60+
• Heart disease
• History of acute kidney injury
• Regular use of NSAIDs or other kidney-affecting medications

Frequently Asked Questions