What GFR Is and Why It Matters

GFR stands for glomerular filtration rate, and it measures how well your kidneys are filtering waste from your blood. The number tells your doctor how many millilitres of blood your kidneys filter each minute, expressed as mL/min/1.73m². A higher GFR means your kidneys are working better; a lower one suggests they may be struggling.

Your doctor uses GFR to catch kidney disease early, monitor existing kidney problems, and adjust medications if your kidneys aren't clearing drugs from your system properly. It's one of the most important numbers in your health record if you have diabetes, high blood pressure, or a family history of kidney problems.

Key Takeaways

  • GFR is calculated from your creatinine level (a waste product in your blood), your age, sex, and race using a formula your lab runs automatically.
  • You don't calculate GFR yourself—your doctor's lab does it when you get a creatinine test, and the result appears on your lab report.
  • A GFR of 60 or higher is generally considered normal kidney function; below 60 may signal chronic kidney disease.
  • The most common formula used in the United States is the MDRD equation, though the newer CKD-EPI equation is becoming standard.
  • Race adjustments in GFR formulas have been controversial, and some labs are removing them; ask your doctor which formula your lab uses.

How the GFR Calculation Actually Works

GFR is calculated using a mathematical formula that takes your serum creatinine level (the amount of creatinine in your blood) and plugs it into an equation along with your age, sex, and sometimes your race. Your lab does this automatically when you have a creatinine test—you don't need to do any math yourself.

The formula estimates how much blood your kidneys filter by measuring creatinine because creatinine is a waste product your muscles produce at a fairly steady rate. If your kidneys aren't filtering well, creatinine builds up in your blood. The formulas used by labs are based on studies of thousands of people and account for the fact that creatinine levels vary naturally by age, sex, and body composition.

Two main formulas are in use. The MDRD equation (Modification of Diet in Renal Disease) has been the standard in most U.S. labs for years. The newer CKD-EPI equation (Chronic Kidney Disease Epidemiology Collaboration) is more accurate, especially at higher GFR values, and is gradually becoming the standard. Some labs now use both or are switching entirely to CKD-EPI. Ask your doctor or lab which one they use.

Understanding Your GFR Number and What It Means

GFR is divided into stages that tell you and your doctor how your kidneys are functioning. A GFR of 90 or higher is considered normal. A GFR between 60 and 89 suggests mild loss of kidney function but is often not a cause for when ready concern if it's stable. Below 60 is generally considered a sign of chronic kidney disease and warrants closer monitoring and possibly treatment.

The lower your GFR, the more advanced the kidney disease. A GFR between 45 and 59 is stage 2 CKD; 30 to 44 is stage 3a; 15 to 29 is stage 3b; and below 15 means your kidneys are failing and you may need dialysis or a transplant. Your doctor will discuss what your specific number means for your health and whether you need treatment or lifestyle changes.

One important note: a single GFR result doesn't always mean you have kidney disease. Your doctor looks at trends over time. If your GFR drops suddenly, that's more concerning than a stable low number. That's why your doctor may order repeat creatinine tests weeks or months apart to see whether your kidney function is getting worse, staying the same, or improving.

What You Need to Know About Race Adjustments in GFR

For decades, GFR formulas included a race multiplier—specifically, they used a higher creatinine threshold for Black patients, which made their GFR appear higher than it actually was. The reasoning was based on the assumption that Black people have more muscle mass on average, but this adjustment has been heavily criticized by nephrologists and researchers as inaccurate and harmful because it delayed diagnosis and treatment in Black patients with kidney disease.

In 2021, the American Society of Nephrology and the National Kidney Foundation recommended removing the race variable from GFR equations. Many labs have already done so, but not all. Some now report two numbers—one with the race adjustment and one without—to help doctors make better decisions. When you get your GFR result, ask your doctor or lab whether a race adjustment was used and what it means for your care.

How to Get Your GFR Calculated

You don't order a GFR test directly. Instead, your doctor orders a serum creatinine test or a basic metabolic panel (BMP), which includes creatinine. You'll have blood drawn at a lab, usually at a hospital, clinic, or independent lab like Quest or LabCorp. The lab measures your creatinine level and automatically calculates your GFR using their formula.

Your GFR result will appear on your lab report, usually listed as "eGFR" (estimated GFR) because it's an estimate, not a direct measurement. Your doctor will review it with you at your next appointment or send you a patient portal message with the results. If you don't see a GFR number on your creatinine report, ask your doctor or lab to calculate it or explain why it wasn't included.

If you have diabetes, high blood pressure, or a family history of kidney disease, your doctor may order creatinine and GFR tests regularly—sometimes once a year, sometimes more often depending on your risk. Keep track of your GFR numbers over time so you and your doctor can spot trends.

Factors That Can Affect Your GFR Result

Several things can temporarily raise or lower your creatinine level and therefore affect your GFR. Intense exercise, high protein intake, and certain medications (like NSAIDs or ACE inhibitors) can raise creatinine. Pregnancy, liver disease, and muscle-wasting conditions can lower it. That's why your doctor may ask you to avoid strenuous exercise for 24 hours before a creatinine test and to fast or follow other prep instructions.

Dehydration can also raise your creatinine level artificially, making your GFR appear lower than it really is. If you're dehydrated when you have your test, your doctor may ask you to repeat it after you've had time to drink fluids and rehydrate. Certain medications, including some blood pressure drugs and antibiotics, can affect kidney function itself, not just the test result, so always tell your doctor about all medications and supplements you take.

When to Talk to Your Doctor About Your GFR

If your GFR is below 60, or if it drops significantly from your previous result, schedule a conversation with your doctor. You don't need to panic—many people live for years with a GFR in the 45 to 59 range—but your doctor may recommend lifestyle changes like reducing salt, managing blood pressure, controlling blood sugar if you're diabetic, or limiting protein intake.

If your GFR is below 30, your doctor will likely refer you to a nephrologist (a kidney specialist) for closer monitoring and a plan to slow the decline. If your GFR falls below 15, you and your nephrologist will discuss options like dialysis or kidney transplant. The earlier kidney disease is caught, the more options you have to slow its progression.

Frequently Asked Questions

Can I calculate my own GFR if I know my creatinine level?

Technically yes, but it's not practical. The formulas are complex and require precise inputs (your exact age, sex, and sometimes race). Your lab's automated system does this when ready and accurately. If you want to understand the math, you can find the MDRD and CKD-EPI equations online, but rely on your lab's official result for medical decisions.

What's the difference between GFR and creatinine?

Creatinine is the waste product your muscles produce; GFR is what the lab calculates from your creatinine level to estimate how well your kidneys filter. A high creatinine usually means a low GFR, but the GFR number is what your doctor uses to assess kidney function because it accounts for your age, sex, and body composition.

Is a GFR of 58 considered normal?

A GFR of 58 is slightly below the normal threshold of 60 and may indicate mild chronic kidney disease. One low result doesn't always mean you have kidney disease—your doctor will look at your trend over time and other factors like protein in your urine. Ask your doctor whether you need follow-up testing or lifestyle changes.

Why did my GFR drop from 72 to 65 in six months?

A drop of 7 points over six months could be normal variation, dehydration at the time of testing, or a sign that your kidney function is declining. Your doctor will consider whether you've had any illness, medication changes, or other factors that could explain it. If the trend continues downward, your doctor may recommend more frequent testing or refer you to a nephrologist.

Does drinking more water improve your GFR?

Staying well-hydrated helps your kidneys work efficiently and can prevent your creatinine from being artificially elevated on a test. However, drinking extra water won't permanently raise a low GFR caused by actual kidney disease. If your kidneys are damaged, the goal is to slow further decline through blood pressure control, blood sugar management, and medication, not to reverse it through hydration alone.