Introduction: Low vitamin D levels (as measured by 25-hydroxyvitamin D [25(OH)D]) are widespread in the population. However, 25(OH)D levels have not been consistently associated with bone mineral density (BMD). It has been suggested that calculation of the free/bioavailable 25(OH)D may give a better correlation with BMD. We examined this hypothesis in a cohort of Malaysian women.
Materials & methods: A cross-sectional study of 77 patients with rheumatoid arthritis (RA) and 29 controls was performed. All subjects were female. Serum 25(OH)D was measured using the Elecsys® Vitamin D total assay. Serum vitamin D binding protein (VDBP) was measured using monoclonal antibody ELISA technique. Free and bioavailable 25(OH)D was calculated using the previously developed equation by modification of the Vermuelen method for free testosterone estimation, and that which was further adapted by Bikle et al [1].
Results: As there were no significant differences between RA patients and controls for VDBP and 25(OH)D, the dataset was analysed as a whole. Calculated free vitamin D by Vermeulen was strongly correlated with calculation by Bickle (r = 1.00, p < 0.001). A significant positive correlation was noted between measured serum total vitamin D, 25(OH)D, with free and bioavailable vitamin D (r = 0.607, r = 0.637, respectively, p < 0.001 for both).
Discussion: Approximately 85?90% of 25(OH)D circulates in association with VDBP, 10?15% is bound less strongly to albumin and only 1% or less is in free form. This unbound form is said to be responsible for the activity of the hormone. Although it has been shown that the free form correlates better with BMD compared to the measured total 25(OH)D [1], this study has not confirmed the finding (Table 1).
Conclusion: In this cohort of Malaysian women, the calculated free and bioavailable 25(OH)D levels do not correlate with BMD and thus do not need to be calculated in routine practice. Should such a calculation be required, the Bickle formula is easier to use but only calculates free 25(OH)D. The Vermuelen formula calculates both free and bioavailable 25(OH)D but is more complex to use.
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