eGFR / CKD Stage

The CKD-EPI 2021 equation estimates GFR using serum creatinine, age, and sex — without a race variable. The 2009 equation included a Black race coefficient that was controversial because race is a social construct, not a biological one. The 2021 equation was refit using diverse data and performs comparably across racial groups. It may slightly overestimate eGFR in Black patients and slightly underestimate in non-Black patients compared to the 2009 equation.

Cystatin C is useful when creatinine-based eGFR may be inaccurate: extremes of muscle mass (bodybuilders, amputees, malnourished), patients on creatine supplements, vegetarian diet, or when confirming CKD diagnosis in borderline cases (eGFR 45-59 without albuminuria). The CKD-EPI creatinine-cystatin C combined equation is more accurate than either alone.

KDIGO classifies CKD by both eGFR (G stages) and albuminuria (A stages): A1 (<30 mg/g, normal), A2 (30-300 mg/g, moderately increased), A3 (>300 mg/g, severely increased). Albuminuria is an independent risk factor for CKD progression, cardiovascular events, and mortality. Always check UACR — two patients with the same eGFR but different albuminuria levels have very different prognoses.