Understanding eGFR and Your Kidney Function Test Results

What eGFR Means and Why It Matters

eGFR stands for estimated glomerular filtration rate. It's a number that shows how well your kidneys are working by measuring how much blood they filter each minute. The "e" at the beginning means "estimated" because doctors calculate it using a formula rather than measuring it directly.

Your kidneys filter waste and extra water from your blood to make urine. This filtering happens in tiny structures called nephrons, which contain even smaller units called glomeruli. When blood passes through these glomeruli, waste products and extra water pass through, while blood cells and proteins stay in your bloodstream. An eGFR test estimates how many milliliters of blood your kidneys filter per minute per 1.73 square meters of body surface area—written as mL/min/1.73m².

The eGFR number matters because it helps doctors understand your kidney function. A higher eGFR means your kidneys are filtering waste better. The normal range for adults is 90 or higher. As the number gets lower, it signals that your kidneys may not be working as well as they should. According to the National Kidney Foundation, about 1 in 7 American adults may have chronic kidney disease, but many don't know it because early stages often have no symptoms.

Understanding your eGFR is important because kidney disease can develop without any noticeable signs. You might feel fine even when your kidneys aren't filtering as efficiently as they should. By knowing your eGFR number, you and your doctor can work together to track changes over time and make decisions about your health care.

Practical Takeaway: Your eGFR is one of the most important numbers on your kidney function report. Ask your doctor what your specific eGFR number is and what it means for your health. Keep track of your eGFR results over time to notice trends—whether it's staying stable, improving, or declining.

How Your eGFR Is Calculated

Your eGFR isn't measured directly. Instead, doctors use a mathematical formula that takes your creatinine level and combines it with other information about your body. Creatinine is a waste product your body makes when muscles break down protein. Your kidneys normally filter creatinine from your blood and send it out in urine.

The most common formula used today is the CKD-EPI equation (Chronic Kidney Disease Epidemiology Collaboration). This formula uses your serum creatinine level—the amount of creatinine in your blood—along with your age, sex, and race. A newer version released in 2021 removed race as a variable, as research showed it didn't accurately reflect kidney function differences between groups.

Here's what happens in the process: First, a lab technician takes a blood sample, usually from your arm. They measure the creatinine level in your blood serum (the liquid part of blood). Then your doctor's office enters your creatinine level, age, and sex into the CKD-EPI formula, which calculates your eGFR. The entire process typically takes a few days from blood draw to receiving results.

The creatinine level is key because it has a direct relationship with kidney function. When your kidneys work well, they filter creatinine efficiently, and your serum creatinine stays low. When kidney function declines, creatinine builds up in your blood, and your serum creatinine rises. However, serum creatinine alone doesn't tell the whole story—that's why the formula includes age and sex. Older adults and men typically have higher creatinine levels even with normal kidney function, so the formula accounts for these differences.

It's worth noting that eGFR is an estimate, not a perfect measurement. Several factors can affect the accuracy of your eGFR, including extreme muscle mass (very muscular people may have higher creatinine), certain medications, liver disease, and diet. Your doctor should consider these factors when interpreting your results.

Practical Takeaway: When you receive your kidney function test results, look for both your creatinine level and your eGFR number. Understanding that eGFR comes from a calculation using creatinine helps you realize why changes in your eGFR reflect changes in your kidney function over time.

Understanding eGFR Stages and What They Mean

Doctors organize kidney function into five stages based on eGFR numbers. Each stage represents a different level of kidney function and helps guide treatment decisions. Here are the five stages:

• Stage 1: eGFR of 90 or higher. This means your kidneys are working normally, even if you have other signs of kidney disease like protein in urine.
• Stage 2: eGFR between 60 and 89. Your kidneys are working reasonably well, but there's mild loss of function. You might have other kidney disease signs.
• Stage 3a: eGFR between 45 and 59. This indicates mild to moderate loss of kidney function.
• Stage 3b: eGFR between 30 and 44. This shows moderate to severe loss of kidney function.
• Stage 4: eGFR between 15 and 29. Your kidneys have severe loss of function. You'll likely need to start preparing for kidney replacement therapy.
• Stage 5: eGFR below 15. Your kidneys are failing or have failed. You'll need dialysis or a kidney transplant to stay alive.

It's important to remember that a single eGFR test doesn't always mean you have kidney disease. Your doctor looks at multiple factors: your eGFR results over several months, whether you have protein or blood in your urine, your blood pressure, and any symptoms you're experiencing. Kidney disease is usually diagnosed when your eGFR is below 60 for at least three months, or when you have other signs of kidney damage regardless of your eGFR number.

Many people with Stage 1 or Stage 2 kidney disease never progress to later stages. With proper management of conditions like high blood pressure and diabetes, and by making healthy lifestyle choices, you can slow or prevent kidney disease from getting worse. According to research published by the National Institutes of Health, people who control their blood pressure carefully reduce their risk of kidney disease worsening by about 50 percent.

The good news is that your kidneys have significant reserve capacity. You can lose a lot of kidney function before you start to notice symptoms. This is why regular kidney function testing matters—it catches changes before you feel sick, giving you time to make changes that can protect your remaining kidney function.

Practical Takeaway: Find out which stage your eGFR puts you in, but don't panic if it's not Stage 1. Talk with your doctor about what your specific stage means for you and what steps you should take to protect your kidney function going forward.

What Can Cause Changes in Your eGFR

Your eGFR isn't necessarily fixed. It can change based on many different factors—some temporary and some long-term. Understanding what causes these changes helps you make sense of your test results.

Short-term factors that can make your eGFR appear lower than it actually is include dehydration, certain medications, and intense exercise. If you haven't been drinking enough water before your blood test, your creatinine level may be higher than usual, making your eGFR appear lower. Some blood pressure medications and antibiotics can temporarily raise creatinine levels. Intense physical activity causes your muscles to break down protein, which increases creatinine. If you had a hard workout the day before your blood test, your creatinine might be elevated. These temporary changes usually go back to normal within days.

Longer-term factors that can cause actual changes in your eGFR include high blood pressure, diabetes, infections, and certain medications used over months or years. High blood pressure and diabetes are the two leading causes of kidney disease in the United States. High blood sugar levels from diabetes can damage the tiny blood vessels in your kidneys over time. High blood pressure puts extra strain on kidney blood vessels, also causing damage. Chronic urinary tract infections or kidney infections can scar your kidneys and reduce function