Harmonization Study Between <scp>LC</scp>‐<scp>MS</scp>/<scp>MS</scp> and Diasorin <scp>RIA</scp> for Measurement of 25‐Hydroxyvitamin D Concentrations in a Large Population Survey

Background Population‐based research on vitamin D has increased dramatically in recent years. Such studies are typically reliant on assay procedures to measure reliable and comparable levels of 25‐hydroxyvitamin D [25( OH )D] concentrations. Methods Concentrations of 25( OH )D 3 and 25( OH )D 2 were measured using LC ‐ MS / MS in 5,915 participants (aged 31 years) of Northern Finland Birth Cohort 1966. Blood samples were assayed in batches over a course of 18 months. As anomalies were present in the measurements, 200 samples were reassayed using Diasorin RIA . Agreement between measurements was assessed by Passing–Bablok regression and limits of agreement (LoA). To harmonize LC ‐ MS / MS with Diasorin RIA measurements, formulae were derived from the LoA. Results Concentrations measured by LC ‐ MS / MS were much higher than those measured by Diasorin RIA , with a mean difference of 12.9 ng/ ml . Constant variation was evident between batch measurements after log transformation. Statistical formula was applied separately for each batch of LC ‐ MS / MS measurements, enabling us to remove both the constant and proportional bias that was evident prior to the transformation. Conclusion Despite the introduction of schemes/programs to improve accuracy of assays to measure 25( OH )D, significant differences can still happen. In these instances, methods to harmonize measurements based on a relatively small number of replicates can be successfully applied to establish confidence and to enable between‐study comparisons.