Variability and reproducibility of circulating vitamin D in a nationwide U.S. population

Abstract

Context: Most studies examining associations between circulating vitamin D and disease are based on a single measure of vitamin D, which may not reflect levels over time, particularly because vitamin D concentrations vary by season. Few studies evaluated how well multiple 25-hydroxyvitamin D [25(OH)D] measures track within the same individual over time.

Objective: This study examined variability and reproducibility of vitamin D by evaluating repeat measurements of plasma 25(OH)D concentrations while accounting for determinants of circulating concentrations including dietary supplement use and latitude of residence from a population of U.S. radiologic technologists.

Design and participants: We analyzed circulating 25(OH)D in blood samples taken from 538 men and women from a prospective, nationwide study at two time points within a 1-yr period, most measured in different seasons. Inter- and intra-individual variability, reliability coefficients, and measurement error were examined.

Results: The spearman rank correlation between two measurements of 25(OH)D concentrations was moderate (r = 0.75, P < 0.001) and did not vary significantly by participant characteristics including age, race, or latitude. The intraclass correlation coefficient was 0.72 (95% confidence interval = 0.68-0.76). The deattenuation factor of plasma 25(OH)D levels was 1.39, suggesting that a single measure of vitamin D on a continuous scale in regression analyses may result in attenuated relationships of about 40%.

Conclusion: Our results suggest that a single blood sample obtained in spring or fall provides a reasonable average for 25(OH)D over a 1-yr period, but additional studies are needed to estimate variability and agreement in plasma 25(OH)D measurements over longer intervals and younger populations.

Fig. 1.

Box-and-whisker plots for distribution of 25(OH)D concentrations at times A and B. The bottom and top horizontal lines of the box represent the 25th and 75th percentiles, respectively. The band near the middle of the box represents the median value. The ends of the whiskers represent the 5th and 95th percentiles. Median (25th, 75th percentiles) was 65.9 (45.8, 85.0) for time A and 69.3 (50.9, 92.6) for time B. Wilcoxon signed rank P < 0.0001.