Least Significant Change (LSC) for Serum Concentrations of 25-hydroxyvitamin D

Background: The Least Significant Change (LSC) should be introduced and considered as a proper method to define the smallest clinically important difference between two consecutive measurements. Methods: The LSC was calculated based on 150 patients with a total 25-hydroxyvitamin D [25(OH)D] IDS-iSYS assay performed in triplicate. The LSC was determined by multiplying the calculated precision error by a factor of 2,77. The study group was additionally divided into subgroups according to gender, age, serum 25(OH)D concentration, and date of assays. Results: The LSC was 4,0 ng/ml (13,2%) for the entire group (n=150; 450 assays) and was not dependent on gender, age of patients or the date of assays (p>0.05). The LSC value depended only on the 25(OH)D concentration value. In the subgroup with vitamin D deficiency (< 20 ng/ml), the obtained LSC value was 2.2 ng/ml (14.7%) and was lower compared to all other groups (p< 0.05 for insufficiency, and p50 ng/ml (n=4; 12 assays), the calculated LSC was 11.8 ng/ml (16.9%) and differed statistically only from the subgroup with vitamin D deficiency (p< 0.005). Conclusion: An absolute LSC of 4.0 ng/ml was calculated for the IDS-iSYS assay used in our study, and should be considered when two (or more) assay results of 25(OH)D performed for a single patient are compared.