Related Experiment Video
Updated: Feb 8, 2026

Operation of a 25 KWth Calcium Looping Pilot-plant with High Oxygen Concentrations in the Calciner
Published on: October 25, 2017
Determination of Free 25(OH)D Concentrations and Their Relationships to Total 25(OH)D in Multiple Clinical
Janice B Schwartz1, J Christopher Gallagher2, Rolf Jorde3,4
1Department of Medicine, University of California, San Francisco, California.
Context:
The optimal measure of vitamin D status is unknown.
Objective:
To directly measure circulating free 25-hydroxyvitamin D [25(OH)D] concentrations and relationships to total 25(OH)D in a clinically diverse sample of humans.
Design:
Cross-sectional analysis.
Setting:
Seven academic sites.
Patients:
A total of 1661 adults: healthy (n = 279), prediabetic (n = 479), outpatients (n = 714), cirrhotic (n = 90), pregnant (n = 20), nursing home resident (n = 79).
Interventions:
Merge research data on circulating free 25(OH)D (directly-measured immunoassay), total 25(OH)D (liquid chromatography/tandem mass spectrometry), D-binding protein [DBP; by radial (polyclonal) immunodiffusion assay], albumin, creatinine, intact parathyroid hormone, and DBP haplotype.
Main Outcome Measures:
Distribution of free 25(OH)D (ANOVA with Bonferroni correction for post hoc comparisons) and relationships between free and total 25(OH)D (mixed-effects modeling incorporating clinical condition, DBP haplotype with sex, race, estimated glomerular filtration rate (eGFR), body mass index (BMI), and other covariates).
Results:
Free 25(OH)D was 4.7 ± 1.8 pg/mL (mean ± SD) in healthy persons and 4.3 ± 1.9 pg/mL in outpatients, with levels of 0.5 to 8.1 pg/mL and 0.9 to 8.1 pg/mL encompassing 95% of healthy persons and outpatients, respectively. Free 25(OH)D was higher in patients with cirrhosis (7.1 ± 3.0 pg/mL; P < 0.0033) and nursing home residents (7.9 ± 2.1 pg/mL; P < 0.0033) than in other groups and differed between whites and blacks (P < 0.0033) and between DBP haplotypes (P < 0.0001). Mixed-effects modeling of relationships between free and total 25(OH)D identified clinical conditions (patients with cirrhosis > nursing home residents > patients with prediabetes > outpatients > pregnant women) and BMI (lesser effect) as covariates affecting relationships but not eGFR, sex, race, or DBP haplotype.
Conclusions:
Total 25(OH)D, health condition, race, and DBP haplotype affected free 25(OH)D, but only health conditions and BMI affected relationships between total and free 25(OH)D. Clinical importance of free 25(OH)D needs to be established in studies assessing outcomes.
Related Concept Videos
Determination of Multiple Dosing Parameters: Steady-State, Minimum and Maximum Concentrations
Conservation of Small Populations
What is Population Genetics?
Determining Order of Reaction
Relationship Formation
Concentration and Rate Law
For example, in a generic reaction aA + bB ⟶ products, where a and b are stoichiometric coefficients, the rate law can be written as:

