Prolonged Frozen Storage of Urine Samples, Jacoline W. Brinkman, Dick de Zeeuw, Jacko J. Duker, Ronald T. Gansevoort, Ido P. Kema, Hans L. Hillege, Paul E. de Jong, and Stephan J.L. Bakker (Departments of 1 Clinical Pharmacology, 2 Internal Medicine, and 3 Pathology and Laboratory Medicine, and the 4 Trial and Coordination Center, University of Groningen and University Medical Center Groningen, Groningen, The Netherlands; * address correspondence to this author at: University of Groningen and University Medical Center Groningen, Department of Internal Medicine, Hanzeplein 1, 9713 GZ, Groningen, The Netherlands; fax 31-50-3639069, e-mail s.j.l.bakker@int.umcg.nl)