Looking beyond linear regression and Bland-Altman plots: a comparison of the clinical performance of 25-hydroxyvitamin D tests.

BACKGROUND: The systematic evaluation of the clinical concordance of various 25-hydroxyvitamin D (25OHD) testing methods is presented. The need for this approach is raised by the discrepancies in the analytical performance of the available assays. METHODS: The analytical and clinical performance of six automated 25OHD assays and an in-house liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was investigated. Leftover serum samples (n=162, SA: n=114) were analyzed and all 21 assay combinations were evaluated. The utility of Cohen's kappa values was assessed by transforming them into minimum percentage agreement (MPA). McNemar's hypothesis test was employed for testing the symmetry of the disagreeing classification outcomes within each method pair. RESULTS: Depending on the assay method, the ratio of results classified as positive (