1. Department of Internal Medicine, St. Franciscus Gasthuis Rotterdam (M.C.C.), 3004 BA Rotterdam, The Netherlands
2. University Medical Center (M.C.C., S.M., D.W.E.), 3508 GA Utrecht, The Netherlands
3. Department of Nephrology, Leiden University Medical Center (C.V.), 2333 ZA Leiden, The Netherlands
4. Department of Clinical Chemistry and Internal Medicine, Erasmus Medical Center (H.J.), 3000 DR Rotterdam, The Netherlands