Estimating Kidney Function and Albuminuria

Our predictors of interest were estimates of GFR by creatinine and cystatin C (GFR_{cystatin C}) and albuminuria expressed as ACR. Blood was collected from participants during an in-home examination after a 12-hour fast. Serum creatinine was measured and calibrated to isotope dilution mass spectrometry-traceable methods.^{21 (link)} Cystatin C was measured by particle-enhanced immunonephelometry (N Latex Cystatin C on the BNII, Formerly, Dade Behring, Now Siemens AG, Munich, Germany).^{13 (link)} Urine albumin was measured by nephelometry using the BNII ProSpec nephelometer (Now Siemens AG), and urine creatinine by the Jaffe method using the Modular-P chemistry analyzer (Roche/ Hitachi, Basel, Switzerland).
We defined stage 3 or higher CKD as an estimated GFR of less than 60 mL/min/1.73 m² using the CKD Epidemiology Collaboration (CKD-Epi) equation for creatinine^{22 (link)}:
$$\begin{array}{l}\text{Estimated}{\text{GFR}}_{\text{creatinine}}=141\times \text{minimum}\\ (\text{serum}\text{creatinine}/\text{k},1)a\times \text{maximum}\\ (\text{serum}\text{creatinine}/\text{k},1)-1.209\times 0.993\\ \text{age}\times 1.018[\text{if}\text{female}]\times 1.159[\text{if}\text{black}]\end{array}$$ and the CKD-Epi cystatin C equation:
$$\text{estimated}{\text{GFR}}_{\text{cystatin}\text{C}}=76.7\times \text{cystain}{\text{C}}^{-1.19}$$
The cystatin C formula was developed from the pooling of several cohorts with GFR measured by iothalamate.^{23 (link)}We defined albuminuria as a spot urine ACR of 30 mg/g or higher.^{1 (link)}