Objective: Vitamin D is important not only for its effect on the homeostasis of calcium, but also for its anti-proliferative, pro-differentiation, pro-apoptotic and immuno-modulating effects in the tissues of living organisms. This work describes the results of serum 25(OH)D (n=2175) and 1alpha,25(OH)2D (n=2271) radioimmunoassays (RIA) conducted at the Institute of Endocrinology in Prague (Czech Republic) during the period of 2004-2006.
Methods: Serum concentrations of vitamin D metabolites were determined by RIA kits from IDS Ltd., Boldon, UK. In a group of 20 healthy volunteers, results obtained using the RIA kit were compared with those obtained after serum extraction with acetonitrile, subsequent chromatographic (RP HPLC) separation of 25(OH)D and its detection in collected fractions using the same RIA kit from IDS.
Results: The mean concentrations +/- S.D. in the samples studied were 76.4 +/- 45.6 nmol/l for 25(OH)D and 88.9 +/- 45.1 pmol/l for 1alpha,25(OH)2D. The mean concentrations of both metabolites were higher in women than in men, but only the difference for 1alpha,25(OH)2D) was statistically significant (p=0.0000). The lowest concentrations of 25(OH)D or 1alpha,25(OH)2D were found in patients with hyperparathyroidism, the highest concentrations those treated with cholecalciferol for osteoporosis and vitamin D deficiency. The relationship between 25(OH)D and 1alpha,25(OH)2D was expressed by the equation 1alpha,25(OH)2D = 71.0845 + 0.1890 * 25(OH)D (n=1065, p=0.0000, R2=0.0343). Based on the RIA results, only 4 % of individuals were inadequately supplied with 25(OH)D, 79 % were supplied adequately and almost 17 % were supplied more than adequately. Similarly, 1alpha,25(OH)2D shows inadequate concentration in 15 % of the individuals tested, 59 % of these individuals were within the reference range and 26 % had values exceeding the upper limit of the reference range. The mean concentrations of 25(OH)D obtained by HPLC separation were in average by 8 % higher than those obtained using only the RIA kit.
Conclusion: Interpretation of studies dealing with vitamin D stores must be based on precise and correct analytical procedures. However, only a few information exists about the influence of other vitamin D metabolites and conjugates on the concentration of immunoanalytically measured serum 25(OH)D or 1alpha,25(OH)2D.