The Glomerular Filtration Rate (GFR) measures how effectively the kidneys clean the blood. It is a key indicator of kidney health, as the kidneys’ primary function is to filter waste products and excess fluid from the body. In clinical settings, GFR is not measured directly but is estimated (eGFR) using a calculation derived from a simple blood test that measures creatinine, a waste product from muscle wear and tear.
Understanding Glomerular Filtration Rate
The filtering process occurs within the kidneys’ functional units, called nephrons, which contain a cluster of blood vessels known as the glomerulus. Glomeruli act like sieves, allowing waste materials, toxins, and excess water to pass through while retaining essential substances like blood cells and proteins. GFR measures the volume of blood these glomeruli filter every minute, typically expressed in milliliters per minute per 1.73 square meters of body surface area (\(\text{mL/min/1.73 m}^2\)).
Because direct GFR measurement is complex, the estimated GFR (eGFR) is calculated using a mathematical formula. This formula incorporates the serum creatinine level along with demographic factors such as age and sex. Creatinine is an effective marker because it is produced at a relatively constant rate by muscle breakdown and is almost entirely removed from the blood by the kidneys. If the kidneys are not filtering efficiently, creatinine accumulates in the blood, resulting in a lower calculated eGFR. Maintaining a healthy filtration rate is important because the kidneys also balance electrolytes, regulate blood pressure, and help produce red blood cells.
Expected GFR Decline With Age
Age is a primary consideration when interpreting GFR results, as a decline in the filtration rate is a normal physiological process. GFR typically peaks in adolescence and young adulthood, with a normal range for healthy younger adults often falling between 90 and 120 \(\text{mL/min/1.73 m}^2\). This rate begins to decrease naturally as early as the third or fourth decade of life, even without the presence of disease.
This decline is largely due to structural changes within the kidneys, including a gradual reduction in the number of functioning nephrons. On average, GFR may decrease by approximately 10 units per decade starting around age 40. For instance, individuals in their 60s have an average eGFR of about 85 \(\text{mL/min/1.73 m}^2\), and those over 70 average near 75 \(\text{mL/min/1.73 m}^2\). While this natural aging process must be acknowledged, a GFR below 60 \(\text{mL/min/1.73 m}^2\) for three months or more remains the diagnostic threshold for chronic kidney disease (CKD) regardless of age.
Interpreting Results: Stages of Kidney Function
The GFR value is the primary factor used to classify the stages of Chronic Kidney Disease (CKD), providing a standardized framework for monitoring and treatment. Kidney disease is divided into five stages based on the degree of function loss, which directly correlates with the eGFR number. As the filtration rate decreases, the severity of the disease increases.
The stages of CKD are defined as follows:
- Stage 1: eGFR of 90 \(\text{mL/min/1.73 m}^2\) or higher, where kidney function is normal but signs of kidney damage, such as protein in the urine, may be present.
- Stage 2: eGFR 60–89 \(\text{mL/min/1.73 m}^2\), indicating mild loss of function.
- Stage 3: Moderate loss of function, subdivided into Stage 3a (eGFR 45–59 \(\text{mL/min/1.73 m}^2\)) and Stage 3b (eGFR 30–44 \(\text{mL/min/1.73 m}^2\)). At this stage, management of complications like anemia and bone disease is often required.
- Stage 4: eGFR between 15 and 29 \(\text{mL/min/1.73 m}^2\), signifying severe loss of function and requiring preparation for future kidney replacement therapy.
- Stage 5: eGFR below 15 \(\text{mL/min/1.73 m}^2\), indicating kidney failure, which necessitates dialysis or a kidney transplant for survival.
Non-Age Factors That Influence GFR
Several non-age-related factors can influence the calculated GFR, sometimes causing temporary or permanent fluctuations. Chronic conditions are major contributors to reduced GFR, with uncontrolled hypertension (high blood pressure) and diabetes mellitus being the most common causes of kidney damage. High blood sugar and pressure damage the small blood vessels in the glomeruli, directly impairing their filtering capacity over time.
Acute issues can also temporarily lower GFR, such as severe dehydration, which reduces blood flow to the kidneys and impairs filtration. Certain medications, including nonsteroidal anti-inflammatory drugs (NSAIDs) and some chemotherapy agents, can also be toxic to the kidneys. Furthermore, the eGFR calculation incorporates variables like sex and body size because they relate to muscle mass, which influences the amount of creatinine produced. Individuals with significantly higher or lower muscle mass than average may have an eGFR that does not accurately reflect their true kidney function. For these populations, healthcare providers may use an alternative marker like cystatin C, a protein less affected by muscle mass and diet, to obtain a more reliable estimate.