Intraindividual Variation in Plasma 25-Hydroxyvitamin D Measures 5 Years Apart among Postmenopausal Women

Background: Current literature examining associations between vitamin D and chronic disease generally use a single assessment of 25-hydroxyvitamin D [25(OH)D], assuming the 25(OH)D concentration of an individual is consistent over time. Methods: We investigated the intraindividual variability between two measures of plasma 25(OH)D concentrations collected approximately five years apart (1997–2000 to 2002–2005) in 672 postmenopausal women participating in the Women's Health Initiative. Plasma 25(OH)D was assessed using the DiaSorin LIAISON® chemiluminescence immunoassay. The within-pair coefficient of variation (CV) was 4.9% using blinded quality control samples. Mean and SDs of 25(OH)D at the two time points were compared using a paired t test. An intraindividual CV and intraclass correlation coefficient (ICC) were used to assess intraindividual variability. A Spearman correlation coefficient (r) assessed the strength of the association between the two measures, and concordance in vitamin D status at two time points was compared. Results: Mean 25(OH)D concentrations (nmol/L) significantly increased over time from 60.0 (SD = 22.2) to 67.8 (SD = 22.2; P < 0.05). The CV was 24.6%, the ICC [95% confidence interval (CI)] was 0.59 (0.54–0.64), and the Spearman r was 0.61 (95% CI = 0.56–0.66). Greater concordance over five years was observed in participants with sufficient compared with deficient or inadequate baseline 25(OH)D concentrations (weighted kappa = 0.39). Reliability measures were moderately influenced by season of blood draw and vitamin D supplement use. Conclusion: There is moderate intraindividual variation in 25(OH)D concentrations over approximately five years. Impact: These data support the use of a one-time measure of blood 25(OH)D in prospective studies with ≤ five years of follow-up. Cancer Epidemiol Biomarkers Prev; 21(6); 916–24. ©2012 AACR.