BACKGROUND Total 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. 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). RESULTS Total-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). CONCLUSIONS In 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.

中文翻译：

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与慢性肾脏疾病相比，肾移植患儿的游离25-羟基维生素D浓度较低。

背景技术总血清25-羟基维生素D [25（OH）D]被认为是维生素D状态的最佳标志物，并在临床实践中常规使用。然而，25（OH）D主要与维生素D结合蛋白（VDBP）结合，据报道，游离25（OH）D和25（OH）D松散结合至白蛋白部分与临床结果更好地相关。方法我们评估了61名儿童的总25（OH）D，游离25（OH）D，游离25（OH）D及其与VDBP和矿物质代谢生物标志物的关系（22 CKD 2-3、18透析和21个移植后）。结果三组患者的总25（OH）D浓度相当（p = 0.09），但游离和生物利用型25（OH）D（游离和白蛋白25（OH）D）的移植物浓度明显较低组（均：p = 0.01）。与CKD和透析患者相比，移植组的VDBP浓度明显更高（p = 0.03）。在所有三个组中，总25（OH）D浓度与测得的游离，计算的和生物利用度25（OH）D正相关。多变量回归分析显示，在透析组中，总25（OH）D是测得的游离25（OH）D浓度的唯一预测因子​​（β= 0.9； R2 = 90％）。在移植组中，可通过总25（OH）D和VDBP浓度预测测得的游离25（OH）D浓度（分别为β= 0.6，-0.6； R2 = 80％）。仅在移植组中观察到甲状旁腺激素与总25（OH）D之间的相关性以及测量和计算的游离25（OH）D之间的相关性（所有：p <0.001）。结论在移植患者中，VDBP浓度明显高于CKD和透析患者，因此，尽管总25（OH）D浓度相当，但游离25（OH）D浓度却较低。我们建议对患有CKD，透析和移植的儿童可能需要使用free-25（OH）D措施，还需要进一步研究以了解其与矿物质代谢标记物的关系。