Challenges in Vitamin D Measurement

Introduction: Vitamin D is needed to maintain calcium concentrations within a narrow physiologic range. Although there is no universal consensus about a treatment cut point for vitamin D, it is suggested that 25 to 35 ng/mL is needed to avoid the adverse effects of deficiency . Vitamin D status is assessed by measuring serum levels of 25-hydroxyvitamin D. Despite the availability of many methods that measure total serum 25-(OH)D levels , their sensitivities and specifications vary considerably. Because different standards and calibrators are used by the different manufactures that makes diagnostic and therapeutic decisions based on absolute cut-off values for vitamin D deficiency very difficult. In addition, studies report testing variability between methods and between laboratories using the same method and classification of patient samples as deficient or non deficient vary by 4% to 32% depending on which assay is used .Another important point is the quality control for each of the assays by manufactures. Most of the commercially available assay kits contain the manufacture’s own quality control material, and there is non or little available from commercial reference sources. Result: To minimize assay to assay variability , the Vitamin D Standardization Program(VDSP) was launched in 2010 in collaboration with other institutes. In Iran , County plans for Standardizing Vitamin D testing are similar to CDC standardization program, which Includes determining a reference measurement system, and calibration of existing commercial assays to reference methods that improves the laboratory performance . Conclusion: This program is essential to patients safety and health, However, manufactures and laboratories will need time to implement the results once they are available. Until then external accuracy based testing can bridge the gap in theVDSP.