Geographical differences in 25(OH)D reference intervals

Vitamin D deficiency has become a widespread public health problem worldwide. In recent years,numerous studies have been conducted to define the reference range for 25(OH)D.The aim was to determine the reference interval of Vitamin D and its metabolically related parameters, including PTH, Ca, P and ALP,using direct and indirect methods for the Southeastern Anatolia and Black Sea regions,which differ significantly in terms of geographical location and dietary habits that greatly influence Vitamin D levels. In the direct method, reference ranges were calculated using non-parametric methods according to CLSI EP28-A3 guidelines. In the indirect method, reference ranges were determined after data filtration and calculated using the Bhattacharya method. In the direct method, the reference interval for 25(OH)D were found to be 8.03–29.44 ng/mL in summer and 5.55–23.07 ng/mL in winter for the Southeastern Anatolia Region, and 6.34–29.69 ng/mL in summer and 6.28–27.34 ng/mL in winter for the Black Sea Region. In the indirect method, the reference interval for 25(OH)D were determined as 7.24–41.69 ng/mL in the Southeastern Anatolia Region and 6.17–42.66 ng/mL in the Black Sea Region. The prevalence of severe Vitamin D deficiency in the reference population during the summer and winter seasons was found to be 7.6 % and 30.5 %, respectively, in the Southeastern Anatolia Region, and 24.1 % and 22.1 %, respectively, in the Black Sea Region. This study has shown that despite variations in reference ranges for laboratory tests due to societal, regional, and seasonal differences, the levels of 25(OH)D were lower than the recommended reference ranges.