Is glomerular filtration rate (GFR) the same as estimated glomerular filtration rate (eGFR)?

No, GFR and eGFR Are Not the Same

GFR (glomerular filtration rate) and eGFR (estimated glomerular filtration rate) are fundamentally different: GFR is the actual, measured rate of kidney filtration, while eGFR is a calculated estimate based on blood biomarkers. 1, 2

Key Distinctions

Measured GFR (mGFR)

• The gold standard for assessing kidney function 2
• Determined using clearance of exogenous filtration markers (iohexol, iothalamate, inulin, EDTA, DTPA) 3, 2
• More expensive, time-consuming, and invasive 2
• Only available in specialized centers 2
• Accurate across all GFR ranges and clinical situations 2
• Less influenced by non-GFR determinants 2
• Can identify early changes in GFR 2

Estimated GFR (eGFR)

• A population-based estimate calculated from serum creatinine and/or cystatin C using validated equations 1, 2
• Inexpensive, widely available, and easily repeatable 2
• Not sufficiently accurate and precise compared to measured GFR 2
• Subject to confounding from non-GFR determinants (muscle mass, diet, medications, chronic illness) 2
• Lags behind actual changes in GFR 2
• Can be inaccurate at the individual patient level 3

Clinical Implications

The terminology matters clinically. KDIGO 2024 guidelines explicitly state that "glomerular filtration rate and clearance are not synonymous" and recommend using "GFR" only when referring to the specific kidney function of glomerular filtration 1, 2.

When eGFR Is Sufficient

• Initial screening and routine monitoring 2
• Most clinical decision-making when accuracy requirements are modest 2
• Use eGFRcr (creatinine-based) as the first-line test 2

When Measured GFR Is Required

Measure GFR when greater accuracy will impact treatment decisions: 2

• Kidney donor candidacy evaluation 2, 4
• Simultaneous kidney transplant decisions 5
• Chemotherapy dosing with narrow therapeutic index 2, 5
• When eGFR is thought to be inaccurate due to:
  • Extremes of muscle mass (sarcopenia, bodybuilders, amputees) 2
  • Severe obesity (BMI >40 kg/m²) 2
  • Catabolic states (cancer, cirrhosis, heart failure) 2
  • Vegetarian/high-protein diets 2
  • Medications affecting creatinine secretion 2

The Accuracy Gap

eGFR equations can have >30% error in 10-50% of patients 6. This occurs because:

• Creatinine is affected by muscle mass, diet, and tubular secretion 3, 2
• Cystatin C is influenced by inflammation, thyroid disease, and corticosteroids 3, 2
• Equations are population averages that may not apply to individual patients 7

The difference between eGFR and mGFR can be clinically significant: eGFR tends to overestimate GFR at lower levels and underestimate at higher levels 8. In kidney donor evaluation, this matters critically—a normal GFR is approximately 120-130 mL/min/1.73 m², and UNOS policy requires confirmation beyond eGFR alone 4.

Practical Algorithm

1. Start with eGFRcr for all patients 2
2. Add cystatin C (eGFRcr-cys) if eGFRcr is likely inaccurate or for confirmatory testing 2
3. Measure GFR when high-stakes decisions depend on precise kidney function assessment 2, 5

Bottom line: While eGFR is adequate for most clinical purposes, it is an approximation. When precision matters for patient safety or major treatment decisions, measured GFR is the only way to know true kidney function.