About Estimated Glomerular Rate (eGFR)
Your kidneys work by filtering your blood to remove waste products and excess water, which are then expelled as urine. The Glomerular Filtration Rate (GFR) is a measure of how well your kidneys perform this filtering task. With an estimated 37 million adults in the U.S. potentially having chronic kidney disease (CKD)—and nearly 90% of them unaware—early detection through GFR assessment is critical for protecting kidney health.
Measuring and Estimating GFR
Obtaining a directly measured GFR (mGFR) is a complex and time-consuming process, making it impractical for routine clinical use. Therefore, healthcare providers rely on a formula to calculate an estimated GFR (eGFR). Since CKD often shows no symptoms until its later stages, a reliable eGFR calculation is vital for early diagnosis.
The most common method for calculating eGFR involves a simple blood test to measure creatinine levels. Creatinine is a waste product from protein digestion and the natural breakdown of muscle tissue. However, factors other than kidney function—such as diet, muscle mass, malnutrition, and other health conditions—can influence creatinine levels.
Differences between eGFR and mGFR
| Estimated GFR (eGFR) | Measured GFR (mGFR) | |
|---|---|---|
| How it works | A calculation estimating how well kidneys filter substances produced by the body, like creatinine (from muscle wear) or cystatin C (a protein). | A direct measurement of how well kidneys filter external agents like inulin (a plant fiber) or iohexol (a medical contrast agent). |
| Availability | Widely available | Not widely available |
| Cost | Less expensive | More expensive |
| Time | Less time needed | More time consuming |
| Accuracy | Estimates can be less accurate in early-stage kidney disease (stages 1 and 2).* | Provides accurate GFR measures, including in early stages. |
| Precision | May not detect subtle or rapid GFR changes. | Can identify early and rapid changes in GFR levels. |
*Other factors like pregnancy, age over 70, unusual muscle mass, liver cirrhosis, nephrotic syndrome, organ transplants, and certain medications can affect eGFR accuracy.
Why eGFR Testing is Done
Since early-stage CKD often has no symptoms, your doctor may recommend an eGFR test if you have a higher risk of developing the disease. Risk factors include:
The development of CKD is typically due to a combination of physical, environmental, and social factors rather than a single cause.
An eGFR test may also be ordered if you exhibit symptoms of later-stage CKD, such as:
- Changes in urination frequency
- Itching or dry skin
- Persistent fatigue
- Swelling in arms, legs, or feet
- Muscle cramps
- Nausea and vomiting
- Loss of appetite
Understanding Your Results
Your eGFR number helps your doctor determine the stage of kidney disease and create an appropriate treatment plan. The results are categorized into stages, from stage 1 (mild) to stage 5 (kidney failure). Discussing your eGFR result with your healthcare provider is the best way to understand your specific situation and next steps.