Samples collection
Fasting venous blood samples were collected into vacutainer tubes containing EDTA and lithium-heparin from patients with CKD and control subjects. For those patients receiving HD, blood samples were collected before and after the midweek dialysis session. All samples were centrifuged at 1500 x g for 10 min immediately after sampling. Then, after the separation process serum and plasma samples were portioned and stored -80 °C until the time of assay.