Department of Internal Medicine, Division of Nephrology, University Medical Center Groningen, University of Groningen, PO box 30, 001 9700, RB, Groningen, The Netherlands. e.m.ettema@umcg.nl. Dialysis Center Groningen, Groningen, The Netherlands. Department of Laboratory Medicine, University Medical Center Groningen, University of Groningen, Groningen, The Netherlands. Department of Epidemiology, University Medical Center Groningen, University of Groningen, Groningen, The Netherlands. Department of Vascular Medicine, University Medical Center Groningen, University of Groningen, Groningen, The Netherlands. Department of Pathology, University Medical Center Groningen, University of Groningen, Groningen, The Netherlands. Department of Internal Medicine, Division of Nephrology, University Medical Center Groningen, University of Groningen, PO box 30, 001 9700, RB, Groningen, The Netherlands.