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-OHD_T for evaluating vitamin D deficiency. In these subjects 25-OHD_T values <15 ng/ml would be more appropriate for defining this deficiency.

INTRODUCTION

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 (25-OHD_T), DBP, albumin, PTH and bone formation (PINP,OC) and resorption (NTx,CTx) markers. Free(25-OHD_F) and bioavailable (25-OHD_B) serum 25-OHD levels were estimated by DBP and albumin determinations and also directly by ELISA (25-OHD_F-2). We analysed threshold PTH values for the different forms of 25-OHD and the correlations and differences according to 25-OHD_T levels <20 ng/ml.

RESULTS

62% of subjects had 25-OHD values <20 ng/ml and also had significantly lower 25-OHD_F and 25-OHD_B 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-OHD_T (15.4 ± 1.4 vs. 18.3 ± 2.7, p < 0.05) and 25OHD_B 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-OHD_T (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-OHD_T for evaluating vitamin D deficiency. In these subjects 25-OHD_T values <15 ng/ml would be more appropriate for defining this deficiency.

Variability of assay methods for total and free PSA after WHO standardization

The variability of total PSA (tPSA) and free PSA (fPSA) results among commercial assays has been suggested to be decreased by calibration to World Health Organization (WHO) reference materials. To characterize the current situation, it is necessary to know its impact in the critical cutoffs used in clinical practice. In the present study, we tested 167 samples with tPSA concentrations of 0 to 20 μg/L using seven PSA and six fPSA commercial assays, including Access, ARCHITECT i2000, ADVIA Centaur XP, IMMULITE 2000, Elecsys, and Lumipulse G1200, in which we only measured tPSA. tPSA and fPSA were measured in Access using the Hybritech and WHO calibrators. Passing-Bablok analysis was performed for PSA, and percentage of fPSA with the Hybritech-calibrated access comparison assay. For tPSA, relative differences were more than 10 % at 0.2 μg/L for ARCHITECT i2000, and at a critical concentration of 3, 4, and 10 μg/L, the relative difference was exceeded by ADVIA Centaur XP and WHO-calibrated Access. For percent fPSA, at a critical concentration of 10 %, the 10 % relative difference limit was exceeded by IMMULITE 2000 assay. At a critical concentration of 20 and 25 %, ADVIA Centaur XP, ARCHITECT i2000, and IMMULITE 2000 assays exceeded the 10 % relative difference limit. We have shown significant discordances between assays included in this study despite advances in standardization conducted in the last years. Further harmonization efforts are required in order to obtain a complete clinical concordance.