Assessing a Method and Reference Material for Quantification of Vitamin D Binding Protein During Pregnancy
Lisa E. Kilpatrick, Ashley S. Boggs-Russell, William C. Davis, Stephen E. Long, James H. Yen, Karen W. Phinney
Previous reports have shown that vitamin D-binding protein (VDBP) concentrations increase with gestational age and reference ranges have been suggested for each trimester. However, these studies used immunoassays which may result in variations between assays from different manufacturers because of differences in antibody epitopes. Measurements of VDBP are also of interest because changes in expression may indicate negative outcomes during pregnancy such as preterm delivery and restricted fetal growth. Additionally, because accurate measurements of 25-hydroxyvitamin D [25(OH)D] depend on complete removal from the binding protein and its concentration increases during pregnancy, there are concerns about the accuracy of 25(OH)D concentrations measurements in pregnant women. To address the need for accurate quantification of VDBP during pregnancy, a method using liquid chromatography-isotope dilution mass spectrometry (LC-IDMS) was developed to quantify VDBP using isotopically labeled peptides as internal standards. This method was used to quantify VDBP in Standard Reference Material (SRM) 1949 Frozen Human Prenatal Serum which was prepared from separate serum pools of women who were not pregnant and women during each trimester of pregnancy. VDBP concentrations were found to be lowest in the serum pool from non-pregnant women and increased in each trimester. These data had good repeatability and were found to be suitable for reference value assignment of VDBP in SRM 1949.