The Glomerular Filtration Rate (GFR) is a standard measure of kidney health, reflecting how effectively the kidneys clear waste products and excess fluid from the bloodstream. By providing a quantifiable metric, GFR allows healthcare providers to assess this filtration function and detect potential kidney impairment early on. Understanding what a normal GFR value is, especially as one gets older, is an important step in managing long-term health.
What GFR Measures and How It Is Calculated
Glomerular Filtration Rate specifically measures the volume of fluid filtered by the glomeruli—the microscopic filtering units within the kidneys—per minute. This filtration process separates waste from the blood, ultimately leading to urine production. A healthy GFR indicates that this filtering process is proceeding at an efficient rate.
Directly measuring GFR is a complicated process, so clinicians rely on an estimated GFR (eGFR), which is calculated using a simple blood test. The primary substance used in this calculation is serum creatinine, a waste product generated from the normal breakdown of muscle tissue. Creatinine is freely filtered by the glomeruli and its blood concentration is inversely related to kidney function, meaning higher levels usually indicate lower GFR.
The estimated GFR is calculated using mathematical formulas that incorporate the serum creatinine level along with demographic factors like age and sex. These formulas, such as the CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) equation, provide a standardized result typically expressed in milliliters per minute per 1.73 square meters of body surface area (mL/min/1.73m²). This estimation serves as a practical, reliable, and widely available proxy for the true filtration rate.
Age-Related Changes in GFR and the 70-Year-Old Benchmark
The GFR naturally declines over a person’s lifespan, a process known as physiologic aging. For a healthy young adult, a GFR value may be around 100 to 120 mL/min/1.73m², but this rate gradually decreases after the age of 40. This decline occurs at an approximate rate of 0.75 to 1.0 mL/min per year.
For a healthy 70-year-old individual, a GFR of 60 mL/min/1.73m² or higher is generally considered within the expected range. Some studies suggest the average estimated GFR for a person in the “70 and over” age group is approximately 75 mL/min/1.73m². A GFR in the range of 60 to 89 mL/min/1.73m² may be acceptable for an older adult who shows no other signs of kidney damage, which is a significant difference from how the same value is interpreted in a younger person.
The age-related decline involves structural changes within the kidney. With advancing age, there is a progressive loss of nephron mass, the functional units responsible for filtration. This loss is often accompanied by reduced renal blood flow and the development of glomerulosclerosis (scarring of the glomeruli).
Other Factors Influencing GFR Results
While age is a primary component of GFR interpretation, several non-age related variables can affect the eGFR calculation and its accuracy. Because the calculation relies heavily on serum creatinine, any factor that influences muscle mass or creatinine production can skew the result. For instance, a person with greater muscle mass, such as a bodybuilder, will naturally have higher creatinine levels, which can lead to an inaccurately low eGFR, even if their kidney function is normal. Conversely, a person with very low muscle mass, such as an individual with sarcopenia or cachexia, will produce less creatinine, potentially leading to an overestimation of their true GFR. Diet also plays a role, as a high-protein intake can temporarily increase creatinine levels, while vegetarian diets may result in lower levels.
Hydration status is another factor, as dehydration can cause a temporary, reversible spike in serum creatinine, lowering the measured eGFR. Certain medications, including nonsteroidal anti-inflammatory drugs (NSAIDs) and some antibiotics, can interfere with kidney function or the production of creatinine, leading to an artificially reduced GFR reading. Furthermore, the eGFR formulas historically included adjustments for race, a controversial practice that is being phased out in clinical practice due to concerns about accuracy and equity.
Interpreting GFR: The Stages of Chronic Kidney Disease
The GFR value is the fundamental measure used to stage Chronic Kidney Disease (CKD) clinically. CKD is diagnosed when a GFR is below 60 mL/min/1.73m² for three months or longer, or when there are other signs of kidney damage like persistent protein in the urine, regardless of the GFR number.
The staging system categorizes the severity of kidney function loss:
- Stage 1: GFR of 90 or greater, requiring evidence of kidney damage (e.g., protein in the urine).
- Stage 3a: GFR 45–59, representing moderate loss of function.
- Stage 3b: GFR 30–44, representing moderate loss of function.
- Stage 5: GFR drops below 15 mL/min/1.73m², indicating end-stage kidney failure.
For a 70-year-old, interpreting the GFR involves considering the rate of decline over time rather than a single absolute number. A stable GFR of 55, while technically Stage 3a, may be less concerning than a rapid drop from 80 to 60 over a six-month period. If a GFR falls into the Stage 3 range or lower, or if there is a significant, sustained decline, follow-up with a nephrologist is generally recommended for further evaluation and management.