Serum 25-hydroxyvitamin D is an important indicator of vitamin D levels. It is associated with human health status, and 25(OH)D levels are negatively associated with all-cause mortality and stroke risk in elderly Chinese [11, 12]. Higher levels of 25(OH)D are associated with lower overall cancer risk [13]. Higher levels of 25(OH)D are also associated with lower all-cause mortality and cardiovascular disease mortality in patients with diabetes [14]. However, vitamin D insufficiency or deficiency is relatively common in Chinese children and adolescents [15] and postmenopausal women [16]. Vitamin D levels can be increased by sunlight exposure or oral vitamin D supplements. Vitamin D levels can be increased by sunlight exposure or oral vitamin D supplements. However, over-supplementation can also be a health threat, and the literature [17] reports that higher lifetime levels of 25(OH)D are associated with an increased risk of leukoaraiosis. Therefore, proper assessment of serum 25(OH)D levels is of great value for health status, disease diagnosis, and prognostic assessment.
Reference intervals are criteria for assessing health status, disease diagnosis, and prognosis. Most clinical decision-making processes are based on the information provided by laboratory reports. Therefore, providing reliable reference intervals is a fundamental task of clinical laboratories [18]. Currently, the reference intervals used in clinical laboratories are mainly derived from the instructions provided by reagent manufacturers, national clinical test protocols, or literature. However, the inconsistency of measurement results among laboratories is prominent due to different testing systems, different testing methods, different reagent manufacturers, and different geographical regions. Therefore, the reference intervals established by each laboratory according to the local population are more valuable for guiding the diagnosis, disease monitoring, and prognosis of diseases. However, the inconsistency of measurement results among laboratories is prominent due to different testing systems, different testing methods, different reagent manufacturers, and different geographical regions. Therefore, the reference intervals established by each laboratory according to the local population are more valuable for guiding the diagnosis, disease monitoring, and prognosis of diseases.
The establishment of reference intervals by the direct method is recommended by the CLSI EP 28-A3C document [10]. However, the direct method has some disadvantages, such as it requires a lot of human and material resources. Also, children may be limited by ethical restrictions, elderly people may be limited by chronic diseases or medication use. Therefore, establishing reference intervals by the direct method is difficult for most laboratories. With the development of information technology, the indirect establishment of reference intervals through a large amount of data in laboratory information systems is accepted by the EP28-A3C document. In this study, the reference intervals for serum 25(OH)D in children in the Zigong region of China were established by statistical analysis based on the results of serum 25(OH)D tests in apparently healthy children in the last 5 years. Serum 25(OH)D levels in children in the Zigong area did not differ statistically significantly between sex, consistent with Xiao Long-necked [19] and others, and were lower in winter and spring and higher in summer and autumn, due to less sunlight and lower UV exposure and reduced skin synthesis in winter and spring than in summer and autumn. 25-hydroxyvitamin D levels are highest in children under 1 year of age and their levels decrease with increasing age, as also observed in Hangzhou, China [20]. The reason for this may be that children under the age of 1 year are mainly fed on breast milk or formula. Also, this group of children will routinely take vitamin D supplementation by oral means and have better compliance with oral vitamin D. As their age increases, children’s physiological needs for vitamin D gradually increase, but vitamin D supplementation is insufficient. Given the interaction of different seasons and ages on serum 25(OH)D levels, this study established reference intervals for serum 25(OH)D in children of different seasons and age groups (Table 5), and serum 25(OH)D levels were higher in children aged 6 ~ 14 years than in Iranian children [21], which may be due to different ethnic groups and different lifestyle habits. The manufacturer’s reagent instructions (> 30 ng/ml as adequate, < 20 ng/ml as deficient, and 20–30 ng/ml as insufficient) referred to the reference interval in this laboratory, the percentage of 25(OH)D insufficiency or deficiency in apparently healthy children amounted to 25.58%. If the health industry-standard (WS/T 677–2020 Screening method for vitamin D deficiency in human population, http://www.nhc.gov.cn/wjw) is used as the judgment standard, the rate of 25(OH)D insufficiency or deficiency is 7.56%, which is mainly distributed in the population aged 6 ~ 14 years old, and the possible reasons are: (1) children in this age group are mostly primary or junior high school students, with increased academic tasks and reduced outdoor sunlight exposure related; (2) children above 6 years of age have increased physiological needs but reduced or missing oral vitamin D preparations. Therefore, time for outdoor exercise should be ensured for primary and secondary school students, serum 25(OH)D levels should be monitored regularly, and vitamin D preparations should be supplemented promptly for those with insufficient vitamin D levels to ensure that their vitamin D is maintained at adequate levels.
In conclusion, this study established reference intervals for serum 25(OH)D in children of different seasons and ages in Zigong, China. When determining the status of vitamin D levels in children, child health practitioners should combine this reference interval with relevant health industry standards to make comprehensive judgments and correct conditions of vitamin D overdose, deficiency, or lack in a timely manner to promote children's health.