The estimated Glomerular Filtration Rate (eGFR) is a calculated measurement used by healthcare professionals to assess kidney function. It estimates how well your kidneys filter waste products from your blood. Derived from a blood test, eGFR is a mathematical approximation, not a direct measurement. Regularly checking eGFR is a standard part of routine health screenings, providing an early indication of kidney health.
Variables Used in the eGFR Calculation
The eGFR calculation relies on serum creatinine levels. Creatinine is a waste product from muscle tissue breakdown and protein digestion. Healthy kidneys filter creatinine from the blood, expelling it through urine. When kidney function declines, creatinine accumulates, indicating reduced kidney filtration.
Age and sex are also factored into the eGFR calculation. Muscle mass and creatinine production vary based on these characteristics. For instance, men typically have more muscle mass than women, influencing their baseline creatinine levels. Creatinine levels can also change with age, even in individuals without kidney disease.
The CKD-EPI Formula Explained
The current standard for calculating eGFR is the CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) 2021 equation. This complex mathematical equation estimates kidney filtration rate using an individual’s serum creatinine level, age, and sex. Medical professionals use specialized calculators to apply this formula.
The CKD-EPI 2021 equation was developed to provide a more precise estimate of glomerular filtration rate, especially at higher levels of kidney function. It relates the logarithm of measured GFR to log-transformed filtration markers, age, and sex. This mathematical model helps healthcare providers evaluate how effectively the glomeruli, the tiny filtering units within the kidneys, are cleaning the blood.
Interpreting Your eGFR Results
Your eGFR number helps determine kidney health. For adults, an eGFR of 90 milliliters per minute per 1.73 square meters (mL/min/1.73 m²) or higher is considered normal. A consistent eGFR below 60 mL/min/1.73 m² for three months or more indicates chronic kidney disease (CKD).
Kidney disease is categorized into stages based on eGFR levels:
Stage 1: eGFR of 90 or higher, often with other signs of kidney damage.
Stage 2: eGFR 60-89, often with mild kidney damage.
Stage 3: Moderate loss of function, divided into 3a (45-59) and 3b (30-44).
Stage 4: Severe loss (15-29).
Stage 5: Below 15, indicating kidney failure.
Evolution of the eGFR Formula
Older eGFR formulas, like MDRD and initial CKD-EPI equations, included a race variable for individuals identified as Black or African American. This adjustment was based on observed statistical differences in average creatinine levels between racial groups.
However, including race in these equations became a concern because race is primarily a social construct, not biological. Its use in clinical algorithms contributed to health disparities, potentially overestimating kidney function in Black patients. To promote equity in diagnosis and care, the National Kidney Foundation (NKF) and the American Society of Nephrology (ASN) convened a task force. This led to the CKD-EPI 2021 equation, which removed the race coefficient, ensuring a single, standardized approach for all individuals.