45: Determining the Agreement Between Commonly Used Assays to Measure 25-Hydroxyvitamin D in Young Children

Determining vitamin D deficiency or insufficiency as defined by the Canadian Paediatric Society (CPS) is accomplished through measuring serum levels of 25-hydroxyvitamin D. Variation in 25-hydroxyvitamin D measurement may lead to misdiagnosis. Several different 25-hydroxyvitamin D assays are used in clinical practice but the comparability of these assays is unclear. The primary objective was to examine the agreement in 25-hydroxyvitamin D concentration measured by three different assays on the same serum sample: Diasorian Liason® (DL), Tandem Mass Spectrometry (MS) and Roche electrochemiluminescence (ECL). Serum samples from a cohort of children under 5 years of age were included in this study. 25-hydroxyvitmain D concentration was measured using DL, MS and ECL at two laboratories. Agreement between assays was explored using Spearman correlations and Bland-Altman plots. CPS endorsed 25-hydroxyvitamin D cut-points, 50 nmol/L and 75 nmol/L, were used to compare differences in classification of vitamin D status. In addition, clinical predictors of 25-hydroxyvitamin D, such as cow's milk intake and vitamin D supplementation, were modeled using three separate multivariate linear regression models with 25-hydroxyvitamin D for each assay as the outcome. The r-squared was compared between models to determine the variation in vitamin D predictors accounted for by each 25-hydroxyvitamin D assay. 264 participants were included in this study. Mean age was 43 months, 56% were male, 64% were European, and mean zBMI was 0.26. The percentages of children falling below each cut-point (50 and 75 nmol/L) were respectively: 7% and 44% for DL; 4% and 35% for MS; and 9% and 36% for ECL. The spearman correlation coefficient was 0.46 for DL and MS, 0.65 for MS and ECL and 0.55 for DL and ECL. The three assays appeared to have poor agreement on Bland Altman plots: DL and MS mean difference was 6.5 (STD 28); MS and ECL mean difference was 1.7 (STD 25); and DL and ECL mean difference was −4.7 (STD 26). The variability in vitamin D clinical predictors explained by each assay was 8.6% using DL, 7.5% using ECL and 1.4% using MS. It appears that there is relatively poor agreement between all three assays commonly used for 25-hydroxyvitamin D measurement. DL and ECL explain more of the variability in vitamin D predictors. More consideration needs to be given to the accuracy of 25-hydroxyvitamin D measurement in children.

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