Seasonal variation of serum 25-hydroxyvitamin D and parameters of bone and mineral disorder in dialysis patients.

Background: Vitamin D deficiency is common among dialysis patients and may impact blood concentrations of calcium, phosphorus, intact parathyroid hormone (iPTH), and alkaline phosphatase (ALP). Seasonal variation of serum 25-hydroxyvitamin D [25(OH)D] concentrations has been well established for the general population; however, less is known about circannual variation in 25(OH)D as well as other parameters of mineral and bone disorder among dialysis patients.

Method: Based on 57,500 serum 25(OH)D measurements collected over two years from January 2009 to December 2010 among 25,025 dialysis patients, we evaluated the circannual variations in serum concentrations of 25(OH)D, calcium, phosphorus, iPTH, and ALP by a linear regression model with a cosinor function for the time period (month). We adjusted for potential confounders including case-mix variables, and ultraviolet index.

Results: Serum 25(OH)D concentrations showed significant circannual variation and mean serum 25(OH)D was 3.2 ng/mL higher in summer than in winter. Furthermore, 25(OH)D concentration increased steadily by 1.3 ng/mL per year. While serum calcium concentrations showed statistically significant but clinically negligible seasonal variation (0.02 mg/dL in peak-trough difference), serum phosphorus did not follow such a pattern. Serum iPTH concentrations also showed a modest seasonal variation with 9% higher values in winter than in summer. Concordantly, ALP concentrations in the winter were 2% higher than in the summer time. Seasonal variation of 25(OH)D was greater in male (vs. female), African-American (vs. non-African-American), and younger (vs. older) dialysis patients.

Conclusion: Serum 25(OH)D and iPTH concentrations show seasonal variation among dialysis patients while the variation in other parameters of mineral and bone disorder was clinically irrelevant, if any. Serum 25(OH)D also showed a gradual increase over time. Clinicians and researchers should be aware of these changes when interpreting laboratory results in dialysis patients.