Vitamin D deficiency presents a significant public health concern, especially in regions where reference intervals from Western populations may not apply due to differences in sun exposure and ethnicity. This study aimed to establish population-specific reference intervals for serum 25-hydroxyvitamin D [25(OH)D] and to determine a deficiency cutoff for healthy adults in Bangladesh. In a cross-sectional design, we assessed serum 25(OH)D and intact parathyroid hormone (iPTH) levels in 125 coastal fishermen (Group 1) and 371 urban residents (Group 2), comprising healthy adults aged 18 years or older. Group 1 served as a reference to establish baseline vitamin D levels, while Group 2 data aided in determining the deficiency cutoff. Measurements were conducted using chemiluminescent immunoassay, and reference intervals were calculated according to Clinical and Laboratory Standards Institute (CLSI) Guidelines C28-A3. The deficiency cutoff was identified at the deflection point of iPTH levels. Results indicate a reference interval for serum 25(OH)D of 15.88–45.27 ng/ml among coastal fishermen. Among urban residents, mean serum 25(OH)D was 21.53 ± 15.98 ng/ml, with iPTH levels showing significant increases below 12.16 ng/ml (95% CI: 11.04–13.28), establishing this as the deficiency cutoff. Urban residents exhibited significantly lower vitamin D levels than coastal fishermen (21.53 ng/ml vs. 27.36 ng/ml, p < 0.001). Limitations include potential selection bias due to convenience sampling and the use of chemiluminescent immunoassay instead of the gold-standard LC-MS/MS assay. This study provides the first population-specific reference intervals for serum 25(OH)D in Bangladesh, accounting for unique sun exposure patterns and ethnic factors, and sets a deficiency threshold at 12.16 ng/ml. These findings are critical for guiding targeted interventions against vitamin D deficiency in this region.

The authors have declared no competing interest.

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The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

The Ethics Review Committee (ERC) of the Centre for Higher Studies and Research, Bangladesh University of Professionals and Diabetic Association of Bangladesh has approved the protocol. Written informed consent was obtained from all participants prior to study enrolment and blood sample collection. The aims and objectives of the study, as well as its procedure, were explained to each participant. Participants were assured that all information and records would be kept confidential and only used for research purposes. Participants had the right to withdraw from the study at any time.

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