1. From the Department of Pediatrics (S.E., U.J.F.T.), University of Groningen, University Medical Center Groningen, the Netherlands
2. Department of Endocrinology (E.G.G., R.P.F.D.), University of Groningen, University Medical Center Groningen, the Netherlands
3. Department of Nephrology (E.G.G., S.J.L.B.), University of Groningen, University Medical Center Groningen, the Netherlands
4. Division of Clinical Chemistry, Department of Laboratory Medicine, Karolinska Institutet, Stockholm, Sweden (U.J.F.T.)
5. Clinical Chemistry, Karolinska University Laboratory, Karolinska University Hospital, Sweden (U.J.F.T.).