A monoclonal antibody sandwich ELISA for vitamin D-binding protein (VDBP) is unaffected by Gc-globulin phenotype peptides and actin and demonstrates reduced levels in sepsis and non-sepsis intensive care patients

The measurement of vitamin D-binding protein (VDBP) by immunoassay has been confounded by variable antibody recognition of the Gc1s, Gc1F and Gc2 phenotypes. This has led to spurious conclusions regarding vitamin D status in different ethnic groups. In order to overcome these problems there is a requirement for VDBP antibodies that are unaffected by phenotype status. Here we report the generation and testing of three monoclonal antibodies to VDBP which recognise linear epitopes and are unaffected by vast molar excesses of synthetic peptides spanning these phenotypic domains. These IgG1 kappa antibodies were purified and biotinylated to allow suitable pairings to develop a sandwich ELISA for circulating VDBP. The VDBP ELISA is unaffected by actin and confirms that VDBP levels are significantly reduced in sepsis patients and non-sepsis intensive care patients compared to normal healthy subjects. Levels of VDBP along with total 25OH vitamin D3 can be used to calculate free 25OH vitamin D3 levels and these compare well with consensus values determined independently. The VDBP ELISA meets acceptable performance criteria and as such can be used in conjunction with total 25OH vitamin D3 to determine the free 25OH vitamin D3 status in various cohorts.