2.1. Laboratory Measurements

Serum 25(OH)D and iPTH levels were measured using an electrochemiluminescence immunoassay (ECLIA) using a cobas^® 8000 modular analyzer (Hitachi High-Technologies Corporation, Tokyo, Japan), and serum total calcium, phosphate, albumin, and magnesium levels were measured by standard methods using a cobas^® 8000 modular analyzer (Hitachi High-Technologies Corporation) in the Department of Clinical Pathology, Faculty of Medicine Siriraj Hospital, Mahidol University, Bangkok, Thailand. The reference ranges of laboratory parameters were as follows: 8.8–10.2 mg/dL for serum calcium; 2.5–4.5 mg/dL for serum phosphate; ≥30–100 ng/mL for serum 25(OH)D; and 10–65 pg/mL for serum iPTH. The intra-assay and interassay coefficients of variation (CVs) of these tests were 0.7% and 0.8% for serum calcium; 0.8% and 1.4% for serum phosphate; 3.3% and 4.5% for serum 25(OH)D; and 0.9% and 1.5% for serum iPTH, respectively.