Interlaboratory comparison with a reference measurement procedure (RMP) for determining 24,25-dihydroxyvitamin D₃ in...

An interlaboratory comparison study was conducted among five laboratories for determining 24,25-dihydroxyvitamin D₃ (24,25(OH)₂D₃) in human serum using liquid chromatography-tandem mass spectrometry (LC-MS/MS) methods. The Centers for Disease Control and Prevention (CDC), Imperial College Healthcare NHS Trust, University College Cork, University of Liège, and University of Washington analyzed 50 single-donor samples and two new Standard Reference Materials (SRMs®). The results from each laboratory were compared with target values assigned by the National Institute of Standards and Technology (NIST) using a reference measurement procedure (RMP) and evaluated using Ordinary Deming linear regression and Bland-Altman analysis. Three of the five laboratory methods provided results that were in good agreement with the NIST RMP results showing linear regression slopes ranging from 0.972 to 1.003 and Bland-Altman mean bias of -0.092 nmol/L, 0.025 nmol/L, and 0.035 nmol/L. Two laboratories demonstrated a significant positive bias with linear regression slopes of 1.158 and 1.214 and Bland-Altman mean bias of 0.162 nmol/L and 0.708 nmol/L. The CDC method is currently used to assign "information only" values for 24,25(OH)₂D₃ in the quarterly distributions of the Vitamin D External Quality Assessment Scheme (DEQAS). Given the agreement (linear regression slope = 0.984, R² = 0.996 and mean bias of -0.092 nmol/L) between the CDC method and the NIST RMP observed in this study, the CDC-assigned 24,25(OH)₂D₃ values in DEQAS may provide a more accurate reference than the current participant consensus mean values.