Clinical Reclassification of Vitamin D Status Across Three Automated Immunoassays After Comparability Study

Background/Objectives: Variability between routine laboratory immunoassays for total 25-hydroxyvitamin D (25(OH) vitamin D) measurement may impact clinical interpretation, particularly around clinical decision thresholds. The aim of the study was to evaluate the analytical comparability and clinical interchangeability between three automated immunoassay platforms using chemiluminescent assays (CLIA) and electrochemiluminescent assay (ECLIA) principles for total 25(OH) vitamin D and to evaluate the assay-dependent impact on patient reclassification. Methods: A total of 83 serum samples were analyzed using three automated immunoassay platforms and the between method comparability was assessed. Additionally, clinical agreement and reclassification rate were evaluated at two clinical decision thresholds, 20 ng/mL for insufficiency and 12 ng/mL for severe deficiency. Results: The comparability analysis revealed a systematic difference between methods, with a high reclassification rate between ECLIA compared to CLIA methods. Additionally, the expanded measurement uncertainty generates a clinically relevant gray zone around the clinically relevant thresholds. Conclusions: Routinely used immunoassays for total 25(OH) vitamin D measurement in clinical laboratories are not fully interchangeable, furthermore the measurement uncertainty contributes to patient reclassification, especially around the threshold of 20 ng/mL.