Blood samples for serum 25(OH)D testing collected during the examination were centrifuged, divided into aliquots, and frozen to −70°C on site and then shipped on dry ice to central laboratories, where they were stored at −70°C until analysis. Serum 25 (OH)D levels were measured by a radioimmunoassay kit after extraction with acetonitrile (DiaSorin, Stillwater, Minnesota) by the National Center for Environmental Health (Atlanta, Georgia). Because risk factors for vitamin D insufficiency have been extensively explored in previous NHANES analyses,30 (link) we focused the current analysis on self-reported age, sex, and race/ethnicity collected during the household interview to evaluate changes in previously documented demographic disparities in vitamin D insufficiency.31 (link),32 Because the month of data collection was not publicly reported in NHANES 2001–2004 (owing to protection of confidentiality), we did not record or control for season.
Protocol detail
Measuring Serum 25(OH)D Levels in NHANES
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Other organizations : University of Colorado Denver, Harvard University Press
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Variable analysis
independent variables
- Self-reported age
- Self-reported sex
- Self-reported race/ethnicity
- Serum 25(OH)D levels
- Season of data collection (not recorded or controlled for)
- Independent variables not explicitly mentioned.
- Dependent variables not explicitly mentioned.
- Positive and negative controls not specified by authors.
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