Understanding GFR and Kidney Function

Your kidneys filter approximately 180 liters of blood every day, removing waste products, balancing electrolytes, and regulating blood pressure. The glomerular filtration rate, or GFR, is the single best measure of how well your kidneys are performing this critical job. Understanding your GFR number helps you detect kidney problems early, when treatment is most effective, and monitor kidney health over time.

This guide explains what GFR measures, how it is calculated from blood tests, what each CKD stage means, the factors that affect your results, and what you can do to protect your kidney function for the long term.

What Is Glomerular Filtration Rate?

Glomerular filtration rate measures the volume of blood that your kidneys filter per minute, expressed in milliliters per minute per 1.73 square meters of body surface area (mL/min/1.73m²). The glomeruli are tiny clusters of blood vessels within each kidney that act as the initial filtration stage, allowing waste products and excess fluid to pass through while retaining blood cells and large proteins.

Each kidney contains approximately one million glomeruli. When these filtering units are damaged or destroyed by disease, the overall filtration capacity of the kidneys decreases. GFR quantifies this capacity, giving doctors a numerical value to assess kidney function and track changes over time.

Why GFR Matters

GFR is the cornerstone of kidney disease diagnosis and staging. A single GFR value tells your healthcare provider whether your kidneys are functioning normally, mildly reduced, moderately reduced, severely reduced, or in failure. It guides decisions about medication dosing (many drugs are cleared by the kidneys and must be adjusted based on GFR), the timing of specialist referrals, and ultimately whether dialysis or transplantation may be needed.

Chronic kidney disease often progresses silently for years before symptoms appear. By the time a person notices fatigue, swelling, or changes in urination, significant kidney function may already be lost. Regular GFR monitoring catches declining kidney function early, allowing interventions that can slow or halt progression.

How GFR Is Calculated

In clinical practice, GFR is almost always estimated from blood tests rather than measured directly. A blood sample measures your serum creatinine level, and this value is plugged into a mathematical equation that accounts for age, sex, and sometimes other variables to produce an estimated GFR (eGFR). Several equations are used in clinical practice, each with different strengths.

The CKD-EPI Equation

The Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation is the most widely recommended formula for eGFR calculation in adults. It was developed from a large, diverse dataset and provides more accurate estimates than older formulas, particularly at higher GFR levels. The 2021 updated version removes the race coefficient and can use serum creatinine alone, cystatin C alone, or both biomarkers together for the most accurate estimate.

• Scr = serum creatinine in mg/dL
• κ = 0.7 for females, 0.9 for males
• α = -0.241 for females, -0.302 for males
• min = the minimum of Scr/κ or 1
• max = the maximum of Scr/κ or 1

Most patients never need to calculate eGFR themselves. Laboratories automatically report eGFR whenever a serum creatinine blood test is ordered. Your healthcare provider interprets the result in the context of your overall health.

The MDRD Formula

The Modification of Diet in Renal Disease (MDRD) formula was the standard for eGFR calculation before CKD-EPI. It uses serum creatinine, age, sex, and previously included a race adjustment. While still seen in some laboratory reports, the MDRD formula is less accurate at GFR levels above 60 mL/min and tends to underestimate kidney function in healthy individuals. Most clinical guidelines now recommend CKD-EPI over MDRD for routine use.

Race-Neutral eGFR (2021 Update)

In 2021, the National Kidney Foundation and American Society of Nephrology released a joint recommendation to adopt race-neutral eGFR equations. The previous CKD-EPI and MDRD formulas included a race coefficient that estimated higher GFR for Black patients. This coefficient was based on population-level data about average muscle mass differences but was criticized for several reasons: it could delay CKD diagnosis in Black patients, race is a social rather than biological variable, and the coefficient did not account for individual variation.

The updated 2021 CKD-EPI equation achieves race-neutral estimation by combining creatinine with cystatin C, a biomarker that is less affected by muscle mass than creatinine. When both markers are available, the combined equation provides the most accurate eGFR estimate across all populations. Laboratories across the United States have been transitioning to the race-neutral CKD-EPI 2021 equation.

CKD Stages by GFR

Chronic kidney disease is classified into five stages based on GFR, with additional subcategories for stages 3 and the albuminuria level. The staging system, developed by KDIGO (Kidney Disease: Improving Global Outcomes), provides a standardized framework for diagnosis, prognosis, and treatment planning.

Stages G1 and G2 are not classified as CKD unless there is also evidence of kidney damage such as albuminuria, structural abnormalities, or a history of kidney transplant. A person with a GFR of 75 and no other markers has mildly decreased kidney function but does not have chronic kidney disease. The same GFR with persistent protein in the urine would be classified as CKD Stage G2.

Understanding Creatinine

Creatinine is a waste product generated by normal muscle metabolism. Your muscles use creatine phosphate for energy, and creatinine is the breakdown byproduct that enters the bloodstream and is filtered out by the kidneys. Because creatinine is produced at a relatively constant rate and is almost entirely cleared by the kidneys, its blood level serves as an indirect measure of kidney filtration.

Normal serum creatinine ranges are approximately 0.7 to 1.3 mg/dL for men and 0.6 to 1.1 mg/dL for women. However, these ranges are broad and heavily influenced by muscle mass. A bodybuilder with a creatinine of 1.4 mg/dL may have perfectly healthy kidneys, while a frail elderly person with a creatinine of 1.0 mg/dL may have significant kidney impairment. This is precisely why eGFR, which adjusts for age and sex, is a more informative metric than serum creatinine alone.

Cystatin C is an alternative biomarker for estimating GFR. Unlike creatinine, cystatin C is produced by all nucleated cells and is not significantly affected by muscle mass, diet, or physical activity. The 2021 CKD-EPI equation combines creatinine and cystatin C for the most accurate eGFR estimate, and cystatin C alone is used when creatinine-based estimates may be unreliable, such as in patients with very high or very low muscle mass.

Factors That Affect GFR

Age and Natural Decline

GFR peaks in the third decade of life at approximately 120 mL/min/1.73m² and then declines at an average rate of about 1 mL/min per year. By age 70, a healthy person may have a GFR around 70 to 80 mL/min. This decline reflects the gradual loss of nephrons (the functional units of the kidney) that occurs naturally with aging. Not every older adult experiences the same rate of decline; some maintain higher GFR well into their 80s, while others decline faster due to hypertension, diabetes, or other conditions.

Muscle Mass, Diet, and Medications

Because creatinine-based eGFR equations assume average body composition, individuals with significantly above-average or below-average muscle mass may have misleading results. High-protein diets can temporarily raise serum creatinine because the body metabolizes dietary protein into creatinine. Creatine supplements, popular among athletes, also elevate serum creatinine without affecting actual kidney function.

Several medications affect serum creatinine independently of kidney function. Trimethoprim (an antibiotic), cimetidine (an antacid), and some HIV medications can block the tubular secretion of creatinine, raising blood levels without changing GFR. Your doctor considers medication use when interpreting eGFR results.

Real-World GFR Examples

Example 1: Healthy Adult

Andrew, age 35, has routine bloodwork done during an annual physical. His serum creatinine is 0.95 mg/dL, and his laboratory report shows an eGFR of 104 mL/min/1.73m². His urinalysis shows no protein. Andrew's kidney function is well within the normal range (G1). He has no signs of kidney damage and no risk factors for CKD. His doctor recommends continuing a healthy lifestyle and rechecking in one to two years.

Example 2: Stage 3a CKD

Brenda, age 58, has a 12-year history of type 2 diabetes and hypertension. Her recent blood test shows serum creatinine of 1.3 mg/dL and an eGFR of 48 mL/min/1.73m². A urine test reveals moderate albuminuria (albumin-to-creatinine ratio of 150 mg/g). Brenda is classified as CKD Stage G3a with A2 albuminuria.

• Her nephrologist starts an ACE inhibitor to protect her kidneys and reduce protein spillage
• Blood pressure target is set below 130/80 mmHg
• HbA1c target is tightened to below 7%
• She is advised to limit sodium to under 2,000 mg per day and moderate protein intake
• GFR will be rechecked every 3 to 6 months to monitor the rate of decline

With careful management of diabetes and blood pressure, Brenda's GFR decline can be slowed significantly. Many patients at this stage maintain stable kidney function for years with proper treatment.

Example 3: Stage 4 CKD

Carlos, age 67, has advanced hypertensive nephrosclerosis and an eGFR of 22 mL/min/1.73m², placing him in CKD Stage G4. He experiences mild fatigue and occasional ankle swelling but otherwise feels well. His nephrologist discusses the trajectory of his kidney function and begins preparing for potential renal replacement therapy.

• Carlos is educated about dialysis options (hemodialysis vs. peritoneal dialysis) and kidney transplantation
• A vascular access site is created for potential future hemodialysis
• His medications are reviewed and adjusted for his reduced kidney function
• Dietary restrictions include limiting potassium, phosphorus, and sodium
• He is referred for transplant evaluation

Not all Stage 4 patients progress to kidney failure. Some stabilize with aggressive risk factor management. However, planning for renal replacement therapy at this stage is standard practice so that treatment can begin without emergency intervention if the kidneys continue to decline.

When to Get Tested

The National Kidney Foundation recommends kidney function screening for anyone with risk factors for CKD, including diabetes, high blood pressure, cardiovascular disease, a family history of kidney disease, obesity, or age over 60. Testing is simple: a blood test for serum creatinine (from which eGFR is calculated) and a urine test for albumin. Together, these two tests provide a comprehensive picture of kidney health.

If you have diabetes, the American Diabetes Association recommends annual screening starting at diagnosis for type 2 diabetes and starting 5 years after diagnosis for type 1 diabetes. If you have hypertension, annual kidney function testing is also recommended. For the general population without risk factors, kidney function is typically checked as part of a comprehensive metabolic panel during routine physicals.

Protecting Your Kidneys

Many of the same lifestyle habits that protect your heart also protect your kidneys. The kidneys receive approximately 20 percent of cardiac output, so cardiovascular health and kidney health are deeply interconnected. Here are evidence-based strategies for maintaining kidney function throughout your life.

• Control blood pressure. Hypertension is the second leading cause of CKD. Keeping blood pressure below 130/80 mmHg reduces the mechanical stress on the delicate glomerular capillaries and slows kidney damage.
• Manage blood sugar. Persistently elevated glucose damages kidney blood vessels. An HbA1c below 7 percent is the standard target for kidney protection in people with diabetes.
• Stay hydrated. Adequate water intake helps the kidneys flush waste products efficiently. Most adults need 1.5 to 2 liters of fluid per day, more in hot climates or during exercise.
• Limit NSAIDs. Nonsteroidal anti-inflammatory drugs like ibuprofen and naproxen reduce blood flow to the kidneys. Occasional use is generally safe, but chronic daily use increases CKD risk.
• Do not smoke. Smoking reduces blood flow to the kidneys, accelerates the progression of existing CKD, and increases the risk of kidney cancer.
• Moderate protein intake. Extremely high protein diets (more than 2 grams per kilogram per day) may accelerate kidney function decline in people with existing CKD. Moderate intake of 0.8 to 1.2 grams per kilogram per day is generally recommended for kidney health.

Common Mistakes to Avoid

• Ignoring risk factors because you have no symptoms. CKD is silent in its early and moderate stages. The absence of symptoms does not mean your kidneys are healthy. If you have diabetes, hypertension, or a family history of kidney disease, get screened regularly.
• Interpreting a single eGFR result without context. One blood test is a snapshot. Dehydration, recent intense exercise, or a high-protein meal the day before testing can all affect results. Trends over multiple tests are far more meaningful than a single value.
• Using creatinine alone to assess kidney function. Serum creatinine is heavily influenced by muscle mass. A low creatinine in a small, elderly person may mask significant kidney impairment. Always look at the eGFR calculation, not just the raw creatinine value.
• Taking kidney supplements or detox products. No herbal supplement, tea, or detox product has been proven to improve kidney function. Some can actually harm the kidneys. Discuss any supplements with your healthcare provider before taking them.
• Skipping medications because you feel fine. Medications like ACE inhibitors, ARBs, and SGLT2 inhibitors protect the kidneys by reducing pressure on the glomeruli. Stopping them because you feel well removes this protective effect and may accelerate kidney damage.

Last updated: February 23, 2026