Abstract
BackgroundTotal serum 25-hydroxyvitamin D [25(OH)D] is considered the best marker of vitamin D status and used routinely in clinical practice. However, 25(OH)D is predominantly bound to vitamin D-binding protein (VDBP), and it has been reported that the free-25(OH)D and 25(OH)D loosely bound to albumin fraction correlates better with clinical outcomes.MethodsWe assessed total-25(OH)D, measured free-25(OH)D, and calculated free-25(OH)D and their relationship with VDBP and biomarkers of mineral metabolism in 61 children (22 CKD 2–3, 18 dialysis, and 21 post-transplant).ResultsTotal-25(OH)D concentrations were comparable across the three groups (p = 0.09), but free- and bioavailable-25(OH)D (free- and albumin-25(OH)D) were significantly lower in the transplant group (both: p = 0.01). Compared to CKD and dialysis patients, the transplant group had significantly higher VDBP concentrations (p = 0.03). In all three groups, total-25(OH)D concentrations were positively associated with measured free-, calculated free-, and bioavailable-25(OH)D. Multivariable regression analysis showed that total-25(OH)D was the only predictor of measured free-25(OH)D concentrations in the dialysis group (β = 0.9; R2 = 90%). In the transplant group, measured free-25(OH)D concentrations were predicted by both total-25(OH)D and VDBP concentrations (β = 0.6, − 0.6, respectively; R2 = 80%). Correlations between parathyroid hormone with total-25(OH)D and measured and calculated free-25(OH)D were only observed in the transplant group (all: p < 0.001).ConclusionsIn transplanted patients, VDBP concentrations were significantly higher compared to CKD and dialysis patients, and consequently, free-25(OH)D concentrations were lower, despite a comparable total-25(OH)D concentration. We suggest that free-25(OH)D measures may be required in children with CKD, dialysis, and transplant, with further research required to understand its association with markers of mineral metabolism.
Highlights
Vitamin D deficiency is prevalent in patients with chronic kidney disease (CKD), including kidney transplant recipients, and is considered to contribute to the pathogenesis of CKDmineral and bone disorder (CKD-MBD) [1,2,3,4]
Gender and ethnicity distribution were comparable between the CKD 2–3, the dialysis, and the transplant groups
Children with kidney transplant had higher body mass index compared to children with CKD 2–3 (p = 0.002) and those on dialysis (p = 0.009)
Summary
Vitamin D deficiency is prevalent in patients with chronic kidney disease (CKD), including kidney transplant recipients, and is considered to contribute to the pathogenesis of CKDmineral and bone disorder (CKD-MBD) [1,2,3,4]. Around 85–90% of total-25(OH)D circulates in the plasma tightly bound to vitamin D-binding protein (VDBP) with 10–15% bound to albumin with much lower affinity, leaving less than 1% in the free form [free-25(OH)D]. This would imply that variations in VDBP concentrations or its affinity for 25(OH)D will alter the relationship between total-25(OH)D concentrations and its free fraction. Methods We assessed total-25(OH)D, measured free-25(OH)D, and calculated free-25(OH)D and their relationship with VDBP and biomarkers of mineral metabolism in 61 children (22 CKD 2–3, 18 dialysis, and 21 post-transplant). We suggest that free-25(OH)D measures may be required in children with CKD, dialysis, and transplant, with further research required to understand its association with markers of mineral metabolism
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