Comparison of total, free and bioavailable 25-OH vitamin D determinations to evaluate its biological activity in healthy adults: the LabOscat study

Determination of different forms of 25-OHD (total, free and bioavailable) in healthy young women does not offer additional advantages over standard 25-OHDT for evaluating vitamin D deficiency. In these subjects 25-OHDT values < 15 ng/ml would be more appropriate for defining this deficiency. ntroduction Determination of 25-OH vitamin D serum levels (25-OHD) constitutes the method of choice for evaluating vitamin D deficiency. However, vitamin D-binding protein DBP) may modulate its bioavailability thereby affecting correct evaluation of 25-OHD status. We analysed the impact of the determination of 25-OHD (total, free and bioavailable) on the evaluation its biologic activity (estimated by serum PTH determination) in healthy young women. Methods 173 premenopausal women (aged 35-45 yrs.) were included. We analysed serum values of total 25-OHD (25OHDT), DBP, albumin, PTH and bone formation (PINP, OC) and resorption (NTx, CTx) markers. Free(25-OHDF) and bioavailable (25-OHDB) serum 25-OHD levels were estimated by DBP and albumin determinations and also directly by ELISA (25-OHDF-2). We analysed threshold PTH values for the different forms of 25-OHD and the correlations and differences according to 25-OHDT levels < 20 ng/ml. Results 62% of subjects had 25-OHD values < 20 ng/ml and also had significantly lower 25-OHDF and 25-OHDB values, with no significant differences in bone markers and PTH values. The PTH threshold value was similar for all forms of 25-OHD (+/- 70 pg/ml). Women with PTH values > 70 had lower 25-OHDT (15.4 +/- 1.4 vs. 18.3 +/- 2.7, p < 0.05) and 25OHDB values (1.7 +/- 0.2 vs. 2.2 +/- 0.09, p < 0.05). The different forms of 25OHD were significantly intercorrelated, with marginal correlations between PTH and 25-OHDT (r = -0.136, p = 0.082). Conclusions Determination of different forms of 25-OHD in healthy young women does not offer additional advantages over standard 25-OHDT for evaluating vitamin D deficiency. In these subjects 25-OHDT values < 15 ng/ml would be more appropriate for defining this deficiency.