Patients with end-stage renal disease (ESRD) experience seasonal variation in vitamin D and intact parathyroid hormone (iPTH) levels, according to a new study.

Kamyar Kalantar-Zadeh, MD, MPH, PhD, of the University of California, Irvine, and colleagues analyzed 57,500 serum 25-hydroxyvitamin D (25(OH)D) measurements collected over 2 years from 25,025 dialysis patients residing in various regions of the United States. Of these, 75% received only in-center hemodialysis, 6% only received peritoneal dialysis, and 19% switched modalities or had some other treatment.

Vitamin D levels varied seasonally, according to results published in Bone. Mean serum 25(OH)D was 3.2 ng/mL higher in summer than in winter. Meanwhile, serum iPTH rose 9% higher in winter than in summer. Geographic area or ultraviolet index did not explain the results. Seasonal variations in alkaline phosphatase, phosphorus, and calcium were not clinically relevant.

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Vitamin D deficiency appears common among dialysis patients, according to the investigators. A greater proportion of patients (74%-77%) had 25(OH)D levels below 30 ng/mL in winter than in summer (67%). Patients who were male, black, or young were more likely to experience seasonal variations in 25(OH)D.

Notably, 25(OH)D concentrations increased steadily by 1.3 ng/mL per year, possibly due to increasing vitamin D supplementation, but the investigators did not have data to confirm this finding.

Clinicians and researchers should be aware of changes in serum 25(OH)D and iPTH when interpreting laboratory results in dialysis patients, Dr Kalantar-Zadeh’s team concluded.

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Kleine CE, Obi Y, Streja E, et al. Seasonal variation of serum 25-hydroxyvitamin D and parameters of bone and mineral disorder in dialysis patients. Bone. DOI:10.1016/j.bone.2019.03.003