STUDY OF SERUM VITAMIN D CONCENTRATIONS IN ELDERLY PATIENTS UNDERGOING PERIODIC HEMODIALYSIS AT FRIENDSHIP HOSPITAL. YHCĐ [Internet]. 2026 May 4 [cited 2026 Sept. 8];67(CĐ5-NCKH). Available from: https://tapchiyhcd.vn/index.php/yhcd/article/view/4977