Estimating Kidney Function and Vitamin D Status

Glomerular filtration rate (GFR) was estimated from the simplified equation developed using MDRD data: eGFR = 186.3 × (serum creatinine in mg/dl)^−1.154 × age^−0.203 × (0.742 for women) × (1.212 if African American).^{(18 (link))} Decreased eGFR was classified as eGFR <60 ml/min/1.73 m². Urine microalbumin was measured with a turbidimetric assay (Albumin; Roche, Germany) using a Hitachi Automatic Analyzer 7600 (Hitachi, Japan). The urine creatinine was measured with a colorimetric assay (CREA; Roche, Indianapolis, IN) using a Hitachi Automatic Analyzer 7600. Elevated uACR was classified as uACR ≥30 mg/g. Serum 25(OH)D levels were measured with a radioimmunoassay (25-hydroxy-vitamin D ¹²⁵I RIA Kit; DiaSorin, Still Water, MN) using a 1470 Wizard Gamma Counter (Perkin Elmer, Turku, Finland). To minimize the analytical variation, serum 25(OH)D levels were analyzed by the same institute, which carried out a quality assurance program through the analysis period. Serum 25(OH)D levels were classified as either vitamin D deficiency [25(OH)D <15.0 ng/ml] and vitamin D sufficiency [25(OH)D ≥15.0 ng/ml].^{(19 (link))}