Aim: Reference intervals/ranges are the most widely used decision-making tool in laboratory medicine and serve as the basis for many of the interpretations of test results. There is no agreement on the reference (normal) ranges for serum total 25-hydroxyvitamin D (25(OH)D) between different societies' guidelines. Therefore, this study was performed to determine our population-based reference ranges of 25(OH)D which is the best indicator of vitamin D status.

Materials and Methods: Results of 8737 adults (1296 male and 7441 female) who were ≥18 years old were obtained retrospectively from the laboratory information system between the dates of 01/05/2015- 01/05/2017. All 25(OH)D levels had been analyzed with liquid chromatography - tandem mass spectrometry. Logarithmic transformation was performed for Gaussian distribution. After the exclusion of extreme values, reference ranges were determined with Bhattacharya method. The need for partitioning reference ranges was considered based on Harris-Boyd model.

Results: After the exclusion of extreme values, remaining participants (8709 adults) median 25(OH)D level was found as 15.6 ng/mL. The determined reference ranges for 25(OH)D for adults were 9.5-20.7 ng/mL. There was no need for partitioning reference range by age group (18-65 or ≥65 years old), gender or seasons.

Conclusion: Determination of the appropriate vitamin D reference intervals and deficiency level is so essential for clinical laboratories to avoid unnecessary testing and consequently unnecessary treatments which can lead to hypervitaminosis D.