The influence of proteoforms: assessing the accuracy of total vitamin D-binding protein quantification by proteolysis and LC-MS/MS.

VDBP proteoforms were not found to cause bias during quantification by LC-MS/MS, thus demonstrating that a family of proteins can be accurately quantified using shared peptides. A reference value was assigned for total VDBP in SRM 1950, which may be used to standardize methods and improve the accuracy of VDBP quantification in research and clinical samples.