https://imcjms.com/public Ibrahim Medical College Journal of Medical Science https://imcjms.com/public/registration/journal_full_text/600 Mon, 04 May 2026 10:27:03 +0000 Abstract Background and objectives: Vitamin D deficiency is a significant public health concern globally including Bangladesh. Currently, Bangladeshi population is being evaluated for vitamin D status using reference interval derived from western studies. Reference interval derived from western populations may not reflect the actual status of vitamin D status of Bangladeshi population due to differences in sun exposure, ethnicity and dietary habits. Therefore, this study aimed to find out the reference intervals and the lower cutoff value of serum 25-hydroxyvitamin D [25(OH)D] deficiency in healthy Bangladeshi adult population. Materials and methods: This cross-sectional study was conducted involving two population groups. Group-1 comprised of adequately sun exposed healthy coastal fishermen and Group-2 included healthy urban dwellers. Group-1 represented an ideal population for assessing the reference serum vitamin D level while Group-2 population was used to detect the cut-off value of vitamin D deficiency level in adult Bangladeshi population. Chemiluminescent microparticle immunoassay (CMIA) was used to estimate serum 25(OH) D. Serum iPTH and other biochemical parameters were analysed by standard methods. The reference interval of vitamin D was determined according to Clinical and Laboratory Standards Institute (CLSI) Guidelines C28-A3. The lower cut-off value of vitamin D deficiency level was determined by detecting deflection point of parathyroid hormone (PTH) level compared to serum 25(OH)D level. Results: Total 125 and 371 individuals were enrolled in Group 1 and Group 2 respectively. The mean age of the Group-1 and 2 study populations were 37.98±11.61 and 44.19 ± 11.48 years respectively. The mean serum vitamin D levels of Group-1 and 2 study population were 27.36±7.27 ng/ml (95% CI: 26.08, 28.65) and 21.53±15.98 ng/ml (95% CI: 19.9, 23.16) respectively. Serum reference interval of vitamin D of healthy adults (Group-1) was found as 15.88 to 45.27 ng/ml. The cut-off value for vitamin D deficiency in Group-2 adult population was 12.16 ng/ml (95% CI: 11.04, 13.28) as depicted by first upward deflection of serum iPTH when serum 25(OH)D level fell below 12.16 ng/ml. Conclusion: The findings suggest that current Western-based vitamin D reference intervals may not be appropriate for the Bangladeshi population. The results of our study would help the clinicians and policymakers in developing strategies to address vitamin D deficiency in Bangladesh.January 2026; Vol. 20(1):006.  DOI: https://doi.org/10.55010/imcjms.20.006 *Correspondence: Wasim Md Mohosin Ul Haque, Department of Nephrology, Bangladesh Institute of Research and Rehabilitation in Diabetes, Endocrine and Metabolic Disorders, Dhaka, Bangladesh. E-mail: wmmhaque@live.com. © 2026 The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License(CC BY 4.0).   Introduction Vitamin D, a crucial fat-soluble vitamin, plays a vital role in various physiological functions, including calcium and phosphorus homeostasis, immune function, and cell growth modulation [1-4]. Consequently, its deficiency can lead to a range of diseases and disabilities [3,5,6]. Despite Bangladesh’s favourable geographical location and climatic condition, the prevalence of vitamin D deficiency remains high even among seemingly healthy individuals [7,8]. Current reference intervals for vitamin D are predominantly derived from studies conducted in Western populations and may not accurately reflect the physiological needs or health outcomes of diverse global populations [9,10]. Variations in genetics, skin pigmentation, dietary habits, and sun exposure significantly impact vitamin D levels, rendering universal reference intervals potentially inappropriate for many non-Western populations. Genetic polymorphisms affecting vitamin D metabolism and receptor activity vary among ethnic groups, leading to different optimal levels of serum 25-hydroxyvitamin D required for health [11,12]. Consequently, a one-size-fits-all approach can result in incorrect diagnoses, unnecessary supplementation, potential toxicity, or under-treatment and persistence of deficiency-related health issues. Therefore, establishing a vitamin D reference interval specific to Bangladeshi population is crucial for accurate diagnosis and effective management of vitamin D deficiency. This study aimed to establish the reference interval of serum 25-hydroxyvitamin D levels of the adult population of Bangladesh by examining two distinct groups: adequately sun-exposed healthy coastal fishermen and urban individuals with limited sun exposure. Also, the study determined the lower cut-off value of vitamin D deficiency level in Bangladeshi adults by detecting the deflection point of serum intact parathyroid hormone (iPTH) level compared to serum 25(OH)D level. Vitamin D deficiency physiologically induces increased synthesis/production of parathyroid hormone.   Materials and methods The study was conducted from January 1, 2017, to December 31, 2021 on adult population living in two geographical locations of Bangladesh. Cross-sectional study design was employed to assess the vitamin D levels in study participants. Study site and population: The study was conducted in two geographical locations: Cox’s Bazar, a coastal district, about 298 km south of Dhaka city along the coast of Bay of Bengal with abundant sunlight; and capital city Dhaka, characterized by limited sun exposure of its inhabitants due to urbanization. Group-1 consisted of healthy adult coastal fishermen aged 18 years and above from Cox’s Bazar, who were regularly and adequately exposed to sunlight and consumed marine fish, a rich source of vitamin D. Adequate sun exposure was defined as exposure to direct sunlight at least for 30 minutes, between 11 am and 2 pm, three times a week [13]. Group-1 individuals (coastal fishermen) were considered as reference population for determining the serum vitamin D reference interval of adult healthy Bangladeshi population. Group-2 comprised of ostensibly healthy urban residents from Dhaka, aged 18 years and above, with limited sun exposure due to lifestyle and environmental factors. Exclusion criteria: Any individual who refused to participate in the study or who were physically and psychologically handicapped, had known acute or chronic illnesses like diabetes mellitus, hypertension, liver disease or gastrointestinal disorders, kidney disease, malignancy, metabolic bone disease, primary hyperparathyroidism or lactose intolerance were excluded. Anyone with relevant biochemical abnormalities and on anti-epileptic drugs was also excluded. For Group-1, any one who was consuming either vitamin D or calcium-containing products or who had ever consumed such drugs in the past was excluded. Sampling technique and sample size: Convenience sampling was used to select participants from both groups. The sample size was determined based on the desired confidence limits and the variability in vitamin D levels observed in preliminary studies. Data collection tools and procedures: Socio-demographic and clinical data were collected using structured questionnaires. Height, weight and blood pressure were measured. Blood samples were collected for estimation of serum 25(OH)D, iPTH levels and relevant biochemical parameters. Collection of blood and biochemical investigations: About 10 ml of venous blood was collected aseptically from each participant after counselling and thorough clinical evaluation. Serum was separated immediately and was preserved at -600C for biochemical analysis. Chemiluminescent micro-particle immunoassay (CMIA) was used to estimate serum 25(OH)D level by Architect analyser [14]. Based on the Endocrine Society guideline [9] (9), vitamin D level was categorised as: adequate/ normal range: 75-100 nmol/L (30-40ng/ml), insufficient: 50 – 74.9 nmol/L (20ng/ml-29.99ng/ml) and deficient: <50 nmol/L (<20ng/ml). Samples were tested for random blood glucose (RBG), serum creatinine, calcium, phosphate (PO4), albumin, alkaline phosphatase and iPTH by Architect autoanalyser at the biochemistry laboratory of BIRDEM. Statistical analyses: Statistical analyses were conducted using descriptive and inferential methods to summarize the data and assess factors influencing vitamin D levels. Chi-square tests, correlation analyses, and regression models were employed to explore these relationships. MedCalc version 14 statistical software was utilized to calculate reference intervals (95%, double-sided) following parametric, non-parametric, and robust methods as outlined in the CLSI C28-A3 guideline [15]. A 90% confidence interval (CI) was used for these reference intervals, applying the appropriate statistical techniques. To determine the lower cut-off value of vitamin D deficiency, the deflection point of intact parathyroid hormone (iPTH) levels was identified using a non-linear model. Piecewise linear regression, applied through the Python plugin, was used to fit two separate lines before and after the deflection point (or “break” point), where the relationship between vitamin D and PTH shifts. The equation used in this case represented as follows: y= { k1​⋅x+y0​−k1​⋅x0 for x 2026 Ibrahim Medical College. All rights reserved.