Glomerular Filtration Rate (GFR) is widely considered the most accurate way to measure how well your kidneys are functioning. In simple terms, it measures the amount of blood passing through the glomeruli—the tiny, ball-shaped clusters of blood vessels that act as filters in your kidneys—each minute.
When the kidneys are healthy, they efficiently filter waste products and excess fluid from the blood. When kidney function declines, the GFR drops, signaling that waste is beginning to build up in the body.
How GFR is Measured and Calculated
While true GFR can be measured directly using specialized markers like inulin (mGFR), it is a complex and time-consuming process. In clinical practice, doctors almost always use eGFR (Estimated GFR).
To calculate eGFR, a simple blood test is performed to measure creatinine, a waste product from muscle breakdown. Because creatinine levels are influenced by more than just kidney function, medical professionals use mathematical formulas (like the CKD-EPI equation) that adjust the result based on:
- Age: Kidney function naturally declines as you get older.
- Sex: Men typically have more muscle mass (and thus higher creatinine) than women.
- Body Size: Larger individuals generally have a higher filtration capacity.
Stages of Chronic Kidney Disease (CKD)
The GFR score is the primary tool used by healthcare providers to stage Chronic Kidney Disease. A normal GFR for a healthy young adult is approximately 100 to 120 mL/min/1.73 m2.
| Stage | GFR Range | Description |
| Stage 1 | 90 or above | Normal kidney function, but some signs of kidney damage (e.g., protein in urine). |
| Stage 2 | 60–89 | Mild loss of kidney function. |
| Stage 3a/3b | 30–59 | Moderate loss of kidney function. This is often when symptoms begin. |
| Stage 4 | 15–29 | Severe loss of kidney function. Preparation for dialysis or transplant may begin. |
| Stage 5 | Less than 15 | Kidney failure (End-Stage Renal Disease). Requires dialysis or transplant. |
Note: A GFR below 60 for three months or longer is the standard clinical definition of Chronic Kidney Disease.
Factors That Can Affect Your Results
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Pregnancy: GFR naturally increases during pregnancy.
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Muscle Mass: Bodybuilders may have high creatinine levels, leading to an underestimated GFR. Conversely, those with muscle-wasting diseases may have an overestimated GFR.
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Diet: Eating a large amount of cooked meat before a blood test can temporarily spike creatinine.
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Dehydration: Severe dehydration can cause a temporary drop in filtration rate.
Early-stage kidney disease rarely has symptoms. Often, a declining GFR is the only early warning sign of a problem. By monitoring GFR, you and your doctor can take steps to slow the progression of kidney damage through blood pressure control, diet adjustments, and medication management.