1 Division of Nephrology, Department of Internal Medicine, University Medical Center Groningen, Groningen, the Netherlands.
2 Division of Clinical Chemistry, Department of Laboratory Medicine, Karolinska Institutet, Stockholm, Sweden; Clinical Chemistry, Karolinska University Laboratory, Karolinska University Hospital, Stockholm, Sweden.
3 Division of Nephrology, Department of Internal Medicine, University Medical Center Groningen, Groningen, the Netherlands. Electronic address: m.h.de.borst@umcg.nl.