eGFR Calculator (CKD-EPI)

How the Kidneys Filter Blood

Your kidneys contain about one million microscopic filtering units called nephrons. Each nephron has a glomerulus—a tiny bundle of capillaries where blood filtration begins. As blood flows through the glomerulus under pressure, water and small molecules (waste products, electrolytes, glucose) pass through the capillary walls into a collecting tubule. Larger molecules like proteins and blood cells stay in the bloodstream.

Glomerular filtration rate (GFR) measures the volume of fluid filtered by all your glomeruli per minute. A healthy young adult filters about 90-120 mL of blood per minute when adjusted for standard body surface area (1.73 m²). This adds up to filtering your entire blood volume dozens of times daily, removing urea, creatinine, excess salts, and other wastes while reabsorbing what your body needs.

When kidney disease damages nephrons, fewer glomeruli remain functional, and GFR declines. Measuring GFR directly is impractical—it requires injecting special markers and timed urine collections. Instead, we estimate GFR from serum creatinine, a waste product of muscle metabolism that kidneys normally filter efficiently. When GFR drops, creatinine accumulates in blood, and the eGFR formula translates this into an estimated filtration rate.

Understanding CKD-EPI and Why It's Used

The CKD-EPI equation replaced the older MDRD formula as the standard eGFR calculation in 2009. Researchers developed it by analyzing creatinine and measured GFR data from more than 8,000 participants in multiple studies. The formula performs better than MDRD at higher GFR levels (above 60 mL/min/1.73m²), reducing the overdiagnosis of kidney disease in people with mildly elevated creatinine but normal kidney function.

CKD-EPI uses a complex equation with different coefficients for sex and race because creatinine production varies by muscle mass. Men typically have more muscle than women, producing more creatinine for the same kidney function. Black individuals on average have higher muscle mass than non-Black individuals, again producing more creatinine. The formula accounts for these differences to avoid misinterpreting higher creatinine from muscle mass as kidney disease.

However, controversy surrounds the race coefficient. Critics argue it's based on problematic assumptions about biological race and can delay diagnosis of kidney disease in Black patients. In 2021, the National Kidney Foundation and American Society of Nephrology recommended developing race-free equations. Some labs now report eGFR both with and without the race coefficient, or use alternative formulas incorporating cystatin C (another filtration marker) that don't require race.

Interpreting Your eGFR and Next Steps

An eGFR of 60 mL/min/1.73m² or higher generally indicates adequate kidney function, though the distinction between G1 (≥90) and G2 (60-89) matters in people with known kidney damage (protein in urine, structural abnormalities). CKD is defined as eGFR below 60 for three months or longer, or evidence of kidney damage regardless of eGFR.

Stage G3a (45-59) represents mild-to-moderate reduction where intervention can slow progression. Focus on controlling blood pressure (target usually <130/80 mmHg), managing diabetes (A1C <7%), avoiding nephrotoxic drugs (NSAIDs, certain antibiotics), and treating underlying causes. Stage G3b (30-44) requires specialist referral, as the risk of cardiovascular events, anemia, and bone disease increases substantially.

At stage G4 (15-29), kidney failure is approaching. Nephrologists begin planning for renal replacement therapy—dialysis or transplant. Stage G5 (<15) is kidney failure; most patients need dialysis to survive, though some with very slow decline can delay for months with intensive medical management. eGFR provides a roadmap: earlier stages offer opportunities to preserve function, while later stages require preparing for the reality that kidneys are failing and replacement therapy will become necessary.