Development and Validation of Novel Free Vitamin D Equations: The Health Aging and Body Composition Study

Vitamin D deficiency is prevalent in 25% of Americans. However, 25(OH)D may not be an accurate measure of vitamin D because the majority (85%–90%) of 25(OH)D is bound to vitamin D binding protein (VDBP), which varies by over 30% across individuals. Free 25(OH)D may be a better measure, but it is difficult to measure accurately and precisely. The existing free 25(OH)D estimating equation does not include VDBP phenotypes; therefore, new equations that include this variable may be more accurate. A total of 370 participants in the Health, Aging, and Body Composition Study, a cohort of healthy community‐dwelling individuals aged 70–79 years old, underwent VDBP and vitamin D metabolite [25(OH)D, 24,25(OH)(2)D, 1,25(OH)(2)D, free 25(OH)D] measurements and were randomly allocated into equation development (two out of three) and internal validation (one out of three) groups. New equations were developed with multiple linear regression and were internally validated with Bland–Altman plots. The mean age was 75 ± 3 years, 53% were female, and the mean measured free 25(OH)D was 5.37 ± 1.81 pg/mL. Three equations were developed. The first equation included albumin, 25(OH)D(3), 25(OH)D(2), VDBP, 1,25(OH)(2)D(3), and 24,25(OH)(2)D(3). The second equation included all variables in Eq. (1) plus VDBP phenotypes. The third equation included albumin, 25(OH)D(3), intact parathyroid hormone, and 1,25(OH)(2)D(3). In internal validation, all three new equations predicted free 25(OH)D values within 30% and 15% of the measured free 25(OH)D concentrations in 76%–80% and 48%–52% of study participants, respectively. Equation (2) was the most precise, with a mean bias of 0.06 (95% limits of agreement −2.41 to 2.30) pg/mL. The existing equation estimated free 25(OH)D within 30% and 15% of measured free 25(OH)D in 43% and 22% of participants, respectively. Free 25(OH)D can be estimated with clinically available biomarkers as well as with more laboratory‐intensive biomarkers with moderate precision. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.