GFR (Glomerular Filtration Rate) is the most reliable indicator of kidney function, measuring how effectively the kidneys filter waste and excess fluid from the blood. It reflects the volume of blood filtered per minute by the glomeruli—tiny filters in the kidneys.
What Is Glomerular Filtration Rate (GFR)?
GFR is a measure of how much blood passes through the glomeruli—the tiny filters in the kidneys—each minute. The glomeruli are microscopic tufts of capillaries housed within each nephron, the functional unit of the kidney. Their primary job is to filter waste products, excess substances, and fluids from the bloodstream, which are then excreted as urine.
Essentially, GFR quantifies the rate at which the kidneys are performing their essential filtration task. It is expressed in milliliters per minute (mL/min) and is adjusted for body surface area, typically standardized to 1.73 m², the average adult body surface area.
Why Is GFR Important?
The GFR is widely considered the best overall indicator of kidney function. It provides a snapshot of the kidneys’ ability to cleanse the blood of wastes and excess fluids. A normal GFR indicates that the kidneys are functioning well, efficiently filtering out toxins and maintaining the body’s delicate balance of electrolytes and fluids. Conversely, a reduced GFR signals impaired kidney function, which can have far-reaching consequences for health.
How Is GFR Measured?
There are two primary ways to determine GFR: direct measurement and estimation.
- Direct Measurement: This involves injecting a special marker substance (such as inulin, iothalamate, or radioisotopes) into the bloodstream and calculating how quickly it is cleared by the kidneys. While accurate, these tests are complex, costly, and impractical for routine clinical use.
- Estimated GFR (eGFR): In everyday practice, GFR is estimated using formulas that incorporate serum creatinine levels, age, sex, and sometimes race or ethnicity. Creatinine is a waste product of muscle metabolism that is filtered out by the kidneys. When kidney function declines, creatinine levels in the blood rise.
Common equations for estimating GFR include the MDRD (Modification of Diet in Renal Disease) Study equation and the CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) equation.
Interpreting GFR Results
A normal GFR varies with age, sex, and body size. Generally:
- Normal GFR for healthy adults: 90–120 mL/min/1.73 m²
- Below 60 mL/min/1.73 m² for three months or more indicates chronic kidney disease (CKD)
- Less than 15 mL/min/1.73 m² is typically classified as kidney failure or end-stage renal disease (ESRD)
It’s important to note that GFR naturally decreases with age, even in people without kidney disease. Children and young adults tend to have higher GFRs, while older adults may have lower GFRs due simply to the aging process.
What Can GFR Tell Us About the Kidneys?
GFR offers multifaceted insights into kidney health:
1. Early Detection of Kidney Disease
A declining GFR is one of the earliest and most reliable signs that the kidneys are not functioning properly. This allows healthcare providers to diagnose kidney disease at an early stage—often before symptoms appear. Early detection is crucial, as timely interventions can slow progression and prevent complications.
2. Staging Chronic Kidney Disease (CKD)
Kidney disease is classified into five stages based on GFR values:
- Stage 1: Kidney damage with normal or high GFR (≥90)
- Stage 2: Mild decrease in GFR (60–89)
- Stage 3a: Mild to moderate decrease (45–59)
- Stage 3b: Moderate to severe decrease (30–44)
- Stage 4: Severe decrease (15–29)
- Stage 5: Kidney failure (<15)
These stages help guide treatment decisions, monitor disease progression, and predict outcomes.
3. Assessing Progression and Prognosis
Serial measurements of GFR over time reveal whether kidney function is stable, improving, or deteriorating. A progressive decline in GFR can signal worsening of underlying conditions such as diabetes or hypertension. The rate of decline helps doctors estimate prognosis and plan for interventions like dialysis or transplantation.
4. Guiding Treatment Choices
Certain medications and therapies may need to be adjusted—or even avoided—based on kidney function. For example, dosages for many drugs are altered in patients with reduced GFR to prevent toxicity. Interventions like blood pressure control, dietary modifications, and management of underlying diseases are tailored according to GFR stage.
5. Evaluating the Effectiveness of Interventions
Improvements or stabilization in GFR after starting a new treatment can indicate its effectiveness. Conversely, a sudden drop in GFR may signal acute kidney injury, medication side effects, or progression of disease. Close monitoring helps clinicians respond promptly to changes.
6. Determining the Need for Renal Replacement Therapy
When GFR falls below 15 mL/min/1.73 m², the kidneys can no longer sustain the body’s needs. At this point, renal replacement therapy—such as dialysis or kidney transplantation—becomes necessary to sustain life.
Limitations of GFR
While GFR is a powerful tool, it is not without limitations:
- Variability: Creatinine-based estimates can be influenced by factors such as muscle mass, diet, medications, and hydration status. Very muscular individuals may appear to have lower kidney function when they do not, and those with low muscle mass may appear to have better function than they actually do.
- Age: Since GFR declines with age, interpreting results in older adults requires careful consideration. A GFR considered low for a young adult may be normal for an elderly person.
- Acute Kidney Injury: GFR estimates are less reliable in rapidly changing conditions, such as acute kidney injury, where serum creatinine may not yet reflect the current state of kidney function.
Beyond Creatinine: Other Markers of Kidney Function
Creatinine is the most common marker used to estimate GFR, but it’s not the only one. Cystatin C, a protein produced by all nucleated cells, has emerged as an alternative marker that is less affected by muscle mass and other confounding factors. Combining creatinine and cystatin C measurements can improve the accuracy of GFR estimation, especially in populations where creatinine-based calculations may be misleading.
REFERENCES
- 1Inker, L.A. & Perrone, R.D. (2021, October 4). Assessment of kidney function.UpToDate. https://www.uptodate.com/contents/assessment-of-kidney-function
- Levey, A. S. and Inker, L. A. (2018). Definition and staging of chronic kidney disease in adults. UpToDate. Retrieved from https://www.uptodate.com/contents/definition-and-staging-of-chronic-kidney-disease-in-adults
- National Library of Medicine (U.S.). Glomerular Filtration Rate (GFR) Test (https://medlineplus.gov/lab-tests/glomerular-filtration-rate-gfr-test/). Last updated 7/6/2023.
- Merck Manual Consumer Version. Chronic Kidney Disease https://www.merckmanuals.com/home/kidney-and-urinary-tract-disorders/kidney-failure/chronic-kidney-disease. Last reviewed 2/2025.
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