P74: CHEMILUMINESCENT IMMUNOASSAY (CIA) AND LIQUID CHROMATOGRAPHY–TANDEM MASS SPECTOMETRY ASSAY (LC/MS‐MS) METHODOLOGY FOR THE DETERMINATION OF VITAMIN D STATUS IN INFANTS AT HIGH RISK FOR DEVELOPING ALLERGIC DISEASES: IS THERE ANY ANALYTICAL BIAS?

Abstract

Given the current interest in vitamin D status and its suggested relationship to immune and metabolic outcomes, the accurate and reliable assessment of 25(OH)D concentrations in both research and clinical settings is imperative. CIA is routinely used in most laboratories whereas LC-MS/MS is defined as the gold standard. However, studies in adults have been performed detecting substantial variation in the measured 25(OH)D3 levels comparing both methods. We are currently conducting the first RCT on vitamin D status in infancy and allergic outcome in early childhood. Vitamin D deficiency is known to be a major problem in Australia and particularly in infants. We therefore used this study cohort as an opportunity to compare CIA and LC-MS/MS methodology. To our knowledge no studies have investigated this before in infants. In this DBPC trial, high risk infants are orally supplemented with either 400 IU vitamin D/day or placebo from birth to 6 months of age. Blood samples are collected at 3, 6 and 12 months of age to determine relationships between oral vitamin D supplementation and blood 25(OH)D concentration, immune cell function responses to allergens and on the development of allergic conditions in early childhood. In each subject 25(OH)D concentration was measured via CIA and LC-MS/MS. We used the Bland Altman Plot for analyzing CIA and LC–MS/MS. 25(OH)D3 levels were analysed by both methodologies (CIA and LC–MS/MS) in 68/120 infants at 3 months, 78/120 at 6 months and 73/120 at 12 months. The highest agreement between both methods was found in 3 months old infants (correlation between difference and mean −0.076), with still good agreement in 12 (correlation between difference and mean −0.251) but no agreement in 6 months old infants. This is the first study in infants comparing the major 25(OH)D assays currently in use and accredited for clinical testing. Compared to adult studies we showed that there is a good agreement between CIA and LC-MS/MS. This knowledge is of particular value as vitamin D levels need to be very accurate and precise to guarantee sufficient bone growth and optimal immune function in utero and infancy. Differences in agreement dependent on the age of the infant may indicate that growth associated with immunological and metabolic changes may influence the assays. Further Paediatric studies comparing both assays in different age groups are warranted.