GFR (Glomerular Filtration Rate): Definition, Measurement & Clinical Stages

Definition

GFR = volume of plasma filtered by glomeruli per unit time (mL/min/1.73 m²). Best index of overall kidney function.

Estimation (eGFR)

• CKD-EPI Creatinine Equation (2021): Preferred; race-free. Uses serum creatinine, age, sex.
• Cystatin C-based or Combined: More accurate in extremes of muscle mass, obesity, elderly.
• Measured GFR (mGFR): Inulin, iothalamate, iohexol clearance – gold standard, used in research/complex cases.

KDIGO CKD Stages (by eGFR)

Stage	eGFR (mL/min/1.73m²)	Description
G1	≥90	Normal/high (with kidney damage)
G2	60–89	Mildly decreased
G3a	45–59	Mild-moderate
G3b	30–44	Moderate-severe
G4	15–29	Severely decreased
G5	<15	Kidney failure

Clinical Significance

• Guides drug dosing (renal adjustment).
• Predicts CVD risk, progression to ESRD.
• Trend > single value; acute drop = AKI.