What Is GFR? A Simple Explanation for Kidney Patients

GFR stands for glomerular filtration rate. It is the best overall measure of how well your kidneys are working. Specifically, it estimates how much blood passes through the tiny filters in your kidneys (called glomeruli) each minute.

Think of it this way: your kidneys are like a filtration system. Every minute, they process a certain volume of blood, removing waste products and excess fluid. Your GFR number tells you how efficiently that filtration system is working. A higher number means better filtration. A lower number means your kidneys are not filtering as effectively as they should.

Doctors use GFR as the primary way to stage chronic kidney disease (CKD) and to track how your kidney function changes over time. It is reported in units of mL/min/1.73m², though you will usually just see a number on your lab report.

In most cases, your GFR is not measured directly. Instead, it is estimated using a blood test that measures your creatinine level. Creatinine is a waste product produced by your muscles. Healthy kidneys filter creatinine out of your blood, so when kidney function declines, creatinine levels rise.

Your doctor's lab uses a formula called the CKD-EPI equation that takes your creatinine level, age, and sex to calculate an estimated GFR (often written as eGFR on your lab results). Some labs may also use a test called cystatin C, which can provide a more accurate estimate in certain situations.

Because GFR is estimated from creatinine, anything that affects your creatinine level can affect your eGFR. This is why a single reading should be interpreted carefully and in context with your overall health picture.

A normal GFR is generally considered to be 90 mL/min or higher. Here is a general guide to what the numbers mean:

• 90 or above: Normal or near-normal kidney function
• 60 to 89: Mildly reduced kidney function
• 30 to 59: Moderately reduced kidney function
• 15 to 29: Severely reduced kidney function
• Below 15: Kidney failure — dialysis or transplant may be needed

It is important to know that GFR naturally declines with age. A GFR of 70 in a healthy 80-year-old may be normal for them, while the same number in a 30-year-old would be concerning. Context matters, and your doctor will interpret your results based on your individual situation.

A low GFR means your kidneys are not filtering blood as effectively as they should be. This can allow waste products to build up in your body, which over time can cause symptoms like fatigue, swelling, nausea, and changes in urination.

However, a single low GFR reading does not automatically mean you have chronic kidney disease. Your doctor will typically want to see at least two low readings over a period of three months or more before making a diagnosis. Temporary factors like dehydration, certain medications, or a recent intense workout can cause a temporary dip in GFR.

If your GFR is consistently below 60, your doctor will likely want to investigate the cause and may refer you to a nephrologist (kidney specialist) for further evaluation and a treatment plan.

Several factors can influence your GFR number, even beyond actual kidney function:

• Age: GFR naturally declines as you get older, even in healthy kidneys.
• Muscle mass: People with more muscle produce more creatinine, which can affect the estimate.
• Diet: Eating a large amount of protein (especially red meat) shortly before a blood test can temporarily raise creatinine and lower your eGFR.
• Hydration: Being dehydrated can cause a temporary decrease in GFR.
• Medications: Some drugs, including common NSAIDs like ibuprofen, can affect kidney function and GFR readings.

This is why your doctor looks at trends over time rather than relying on a single number. If your GFR fluctuates slightly between tests, that is usually normal and not a cause for alarm.

You should discuss your GFR with your doctor if your eGFR is below 60, if your GFR has dropped significantly since your last test, or if you have other risk factors for kidney disease such as diabetes, high blood pressure, or a family history of kidney problems.

Remember that early detection gives you the best chance of slowing kidney disease progression. Many people with early-stage CKD can maintain stable kidney function for years with proper management, including blood pressure control, dietary changes, and avoiding medications that stress the kidneys.

If you are unsure what your GFR number means for your specific situation, ask your doctor to explain it in the context of your overall health. You have every right to understand your own lab results.