Vitamin D Status and Routine Laboratory Data-Derived 25(OH)D Distributional Benchmarks in Adults from Şanlıurfa, Türkiye: Age, Sex, and Seasonal Variation

Background: Serum 25-hydroxyvitamin D [25(OH)D] interpretation requires clear distinction between epidemiological thresholds, routine-data distributions, and clinical decision limits. This study evaluated vitamin D status and clinically pre-filtered routine laboratory data-derived 25(OH)D distributional benchmarks among adults from Şanlıurfa, southeastern Türkiye, according to sex, age, and season. Methods: This retrospective single-center routine laboratory database study included adults aged 18–65 years tested between 1 January and 31 December 2025, at the Medical Biochemistry Laboratory of Şanlıurfa Mehmet Akif İnan Training and Research Hospital. After eligibility screening, duplicate removal, analytical screening, and predefined clinical pre-filtering, 48,826 participant-level records were analyzed. Serum 25(OH)D was measured using the Elecsys Vitamin D total III electrochemiluminescence immunoassay on a cobas e 801 analyzer/module. The primary distributional estimate was the nonparametric 2.5th–97.5th percentile range. Results: Median age was 38 [28–49] years, and 35,043 records were from female participants (71.8%). Median serum 25(OH)D was 12.74 [8.28–19.00] ng/mL. Vitamin D deficiency, severe deficiency, insufficiency, and sufficiency were observed in 38,072 (78.0%), 17,163 (35.2%), 8235 (16.9%), and 2519 (5.2%) records, respectively. Lower 25(OH)D concentrations and higher deficiency prevalence were observed among females, younger adults, and winter/spring samples, with small-to-modest effect magnitudes. The clinically pre-filtered routine-data 2.5th–97.5th percentile range was 3.46–35.50 ng/mL. Conclusions: Low 25(OH)D status was widespread among routinely tested adults in Şanlıurfa. The derived range should be interpreted only as a local routine-data distributional benchmark for the tested population, not as a healthy-volunteer reference interval, clinical sufficiency threshold, treatment threshold, or clinical decision limit.