In more detail
The glomeruli are the kidneys' filtering units; GFR is the volume of blood plasma they clear each minute, usually scaled to a standard Body surface area (BSA) of 1.73 m². Measuring it directly is laborious, so laboratories report an estimate (eGFR) from blood creatinine. The CKD-EPI 2021 equation is 142 × min(Scr/κ, 1)^α × max(Scr/κ, 1)^−1.200 × 0.9938^age, multiplied by 1.012 for women, with creatinine in mg/dL, κ = 0.7 (women) or 0.9 (men) and α = −0.241 (women) or −0.302 (men). The 2021 version removed the earlier race coefficient; cystatin C is an alternative marker.
Why it matters
Kidney function is staged by GFR: G1 is 90 or more, G2 60 to 89, G3a 45 to 59, G3b 30 to 44, G4 15 to 29 and G5 below 15. Chronic kidney disease is diagnosed when GFR stays below 60 for more than three months or other signs of kidney damage are present. Creatinine depends on muscle mass and diet, so eGFR can mislead in very muscular or very frail people. Some drug doses use the Cockcroft-Gault creatinine clearance instead, which is a different number. Interpretation belongs to a clinician.
Example
Same creatinine, 1.0 mg/dL (88.4 µmol/L), CKD-EPI 2021:
Man, 50: 91.7 mL/min/1.73 m² (G1)
Woman, 50: 68.6 (G2)
Man, 75: 78.5 (G2)
Man, 70, creatinine 1.5: 49.8 (G3a)