1. Division of Nephrology, Department of Internal Medicine, University of Groningen, University Medical Center Groningen, Groningen, the Netherlands
2. Department of Laboratory Medicine, University of Groningen, University Medical Center Groningen, Groningen, the Netherlands
3. Division of Endocrinology, Department of Internal Medicine, University of Groningen, University Medical Center Groningen, Groningen, the Netherlands
4. Certe, Department of Clinical Chemistry, Martini Hospital, Groningen, the Netherlands