Outline of this thesisThis thesis focuses on measurements of circulating MGP species and their potential diagnostic and clinical value. Chapter 1 gives an introduction of MGP and vitamin K. In Chapter 2, the mono-antibody MGP ELISA measuring total uncarboxylated MGP (t-ucMGP) levels is presented and characterised, with application of t-ucMGP measurements in patient populations with cardiovascular disease (angioplasty and aortic valve stenosis) and vascular calcification (hemodialysis and calciphylaxis). Further investigation of t-ucMGP as a biomarker in dialysis patients is described in Chapters 3 and 4. Chapter 3 describes the correlation between t-ucMGP levels and the extent of coronary artery calcification in hemodialysis patients. In Chapter 4, the association between t-ucMGP levels and (cardiovascular) mortality in a large Dutch cohort of hemodialysis and peritoneal dialysis patients is investigated. In Chapter 5, we compared the four available MGP assays, including two novel dual-antibody MGP ELISA. All assays were applied in patients with cardiovascular disease, rheumatic diseases, and end-stage renal disease. To determine which assay would be suited to assess vascular vitamin K status, the effect of changes in vitamin K status on circulating MGP species was also investigated. For this, the assays were applied in two groups of healthy subjects with opposite vitamin K status: subjects receiving vitamin K supplementation and those receiving vitamin K antagonists. In Chapter 6, we investigated the functions of MGP in humans by phenotypic characterization of a newly-identified Dutch patient with Keutel syndrome. In order to further elucidate the difference in phenotype between MGP deficient mice and men, we characterized the circulating MGP species of this patient and his first-degree relatives. Additionally, MGP species were determined in tissues from the first described Keutel syndrome patient in the literature. Chapter 7 describes the investigation of the vitamin K intake and the hepatic and extra-hepatic vitamin K status in a Dutch cohort of hemodialysis patients.