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Activity Number: 615
Type: Contributed
Date/Time: Thursday, August 2, 2012 : 8:30 AM to 10:20 AM
Sponsor: Biometrics Section
Abstract - #305918
Title: Statistical Analysis of Seasonal Variation in Vitamin D Concentrations in the MESA Study
Author(s): Michael Sachs*+ and Ian DeBoer
Companies: and Kidney Research Institute
Address: 201 Galer Street, Seattle, WA, 98109, United States
Keywords: seasonal variation ; prediction ; sinusoidal ; chronobiology

Low circulating concentrations of 25-hydroxyvitamin D (25D) are associated with adverse health outcomes. However, 25D concentrations vary seasonally with varying exposure to sunlight. Therefore, single measurements of 25D may poorly reflect typical concentrations. It is usually impractical or infeasible to repeatedly measure 25D concentrations, thus several authors have proposed to use a sinusoidal model to account for seasonal variation in 25D. A simple time transformation makes the sinusoidal model estimable with standard regression techniques, allowing for covariate effects on both the mean and the amplitude of the curve. We estimate individuals' average or cumulative exposure on the basis of the estimated sinusoidal model and covariate information. For the first time we can evaluate the within-subject performance of the sinusoidal predictive model, and assess whether it can be used to reduce misclassification in the exposure when only one measurement is available. This was made possible by taking second measurements of 25D on a subset of 373 out of 6814 individuals enrolled in the Multi-Ethnic Study of Atherosclerosis.

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