Is Vitamin D Dialysable?
Vitamin D is not significantly dialysable during hemodialysis or peritoneal dialysis procedures, as only a very small percentage of body vitamin D is available for removal through dialysis. 1
Understanding Vitamin D in Dialysis Patients
Forms of Vitamin D and Their Dialysability
Native Vitamin D (25-hydroxyvitamin D):
- Highly protein-bound (to vitamin D binding protein)
- Large molecular weight complex
- Not significantly removed during dialysis sessions
Active Vitamin D (1,25-dihydroxyvitamin D/calcitriol):
- Also protein-bound
- Not effectively cleared by dialysis
Evidence from Guidelines
The KDIGO guidelines acknowledge that "the percentage of body calcium that is dialyzable is very small" 1. This statement reflects the broader understanding that vitamin D, which is integral to calcium metabolism, is similarly not significantly removed during dialysis.
The K/DOQI clinical practice guidelines further support this by focusing on vitamin D supplementation rather than concerns about its removal during dialysis 1. The guidelines recommend specific dialysate calcium concentrations (2.5 mEq/L) with the understanding that vitamin D therapy can continue without significant loss during dialysis.
Clinical Implications
Vitamin D Deficiency in Dialysis Patients
Vitamin D deficiency is extremely common in dialysis patients for several reasons:
- Reduced sun exposure
- Dietary restrictions
- Impaired vitamin D synthesis and metabolism
- Loss of renal tissue needed for 1-hydroxylation 2
Supplementation Considerations
Since vitamin D is not significantly removed by dialysis:
- Supplementation can be scheduled without concern about timing relative to dialysis sessions
- Dosing does not need to be adjusted to account for dialysis-related losses
- Both ergocalciferol and cholecalciferol can be used effectively in dialysis patients 3
Monitoring Requirements
- Regular monitoring of 25(OH)D levels (every 3-6 months)
- Monitoring of calcium and phosphorus levels
- Adjustment of active vitamin D analogs based on PTH levels
Special Considerations
Vitamin D Toxicity
In rare cases of vitamin D toxicity with severe hypercalcemia, peritoneal dialysis has been used as a management strategy 4. This represents an exceptional circumstance rather than routine care and suggests that only in extreme overdose situations might dialysis be considered for vitamin D removal.
Calcium Balance During Dialysis
While vitamin D itself is not significantly dialysable, the dialysate calcium concentration affects calcium balance, which indirectly impacts vitamin D metabolism and requirements. The recommended dialysate calcium concentration is 2.5 mEq/L (1.25 mmol/L) 1.
Conclusion
The clinical evidence consistently indicates that vitamin D is not significantly removed during dialysis procedures, allowing for consistent supplementation strategies in dialysis patients without concerns about dialysis-related losses.