The vitamin D spectrum: insights into 25(OH)D and VDBP in chronic kidney disease and post-transplant

CKD progression is associated with declining vitamin D levels, which correlate with various comorbidities. Total serum 25-hydroxyvitamin D (Total-25(OH)D) may not accurately reflect vitamin D deficiency in these patients. This study aimed to evaluate the vitamin D spectrum in CKD and kidney transplant recipients (KTRs). We recruited 38 stage 3 & 4 CKD patients and 38 patients with ESRD, who were followed for six-months post-KT. Total-25(OH)D and VDBP were measured using ELISA. Free and bioavailable-25(OH)D concentrations were calculated using Bikle formula. ESRD patients had lower total-25(OH)D, free-25(OH)D, and bioavailable-25(OH)D, compared to stage 3 & 4 CKD patients. VDBP levels showed no significant difference. Six months post-KT, a continued decline in free and bioavailable-25(OH)D was observed, whilst VDBP was increased. No significant change was found in total-25(OH)D levels. On regression analysis, bioavailable-25(OH)D and creatinine could independently predict CKD stage (based on eGFR). Total-25(OH)D, free-25(OH)D, and bioavailable-25(OH)D showed significant decline, with increasing severity of CKD. Notably, even after KT, they did not show significant improvement. Elevated VDBP post-KT probably contributed to continued decline in free and bioavailable-25(OH)D. Further research with extended follow-up is warranted to assess whether these alterations have a significant impact on long-term patient outcomes.