Estimating Kidney Function: Serum Creatinine and Cystatin C

Serum Cr and CysC levels were measured using a Beckman Coulter AU5800. Serum Cr levels were measured with the isotope dilution mass spectroscopy-traceable Jaffe method using picric acid. Serum CysC levels were measured by turbidity analysis. The CKD-EPI eGFR equations, the 2021 CKD-EPI creatinine equation (eGFR_Cr), 2012 CKD-EPI cystatin C equation (eGFR_CysC), and 2021 CKD-EPI creatinine-cystatin C equation (eGFR_Cr-CysC) were as follows: eGFR_Cr = 142 × min (S_Cr/κ, 1)^a × max (S_Cr/κ, 1)^−1.200 × 0.9938^age × (1.012 if female), eGFR_CysC = 133 × min (S_Cys/0.8, 1)^−0.499 × max (S_Cys/0.8, 1)^−1.328 × 0.996^age × (0.932 if female), and eGFR_Cr-CysC = 135 × min (S_Cr/κ, 1)^b × max (S_Cr/κ, 1)^−0.544 × min (S_Cys/0.8, 1)^−0.323 × max(S_Cys/0.8, 1)^−0.778 × 0.9961^age × (0.963 if female). S_Cr is the serum Cr level; S_Cys is the serum CysC level; κ is 0.7 for females and 0.9 for males; a is − 0.241 for females and − 0.302 for males, and b = − 0.219 for females and − 0.144 for males^{3 (link),4 (link),10 (link)}.