E-Mail karger@karger.com Original Report: Patient-Oriented, Translational Research Am J Nephrol 2015;41:248–256 DOI: 10.1159/000382081 Urine and Plasma Osmolality in Patients with Autosomal Dominant Polycystic Kidney Disease: Reliable Indicators of Vasopressin Activity and Disease Prognosis? Niek F. Casteleijn   a Debbie Zittema   a Stephan J.L. Bakker   a Wendy E. Boertien   a Carlo A. Gaillard   a Esther Meijer   a Edwin M. Spithoven   a Joachim Struck   b Ron T. Gansevoort   a   a  Department of Nephrology, University Medical Center Groningen, University of Groningen, Groningen, The Netherlands; b  Thermo Fisher Scientific, Hennigsdorf/Berlin, Germany (p = 0.3), in contrast to plasma osmolality (R = 0.52, p < 0.001) and copeptin concentration (R = 0.61, p < 0.001). Fifty-five patients were followed for 2.8 ± 0.8 years. Baseline plasma and urine osmolality were not associated with change in eGFR (p = 0.6 and p = 0.3, respectively), whereas baseline co- peptin concentration did show an association with change in eGFR, in a crude analysis (St. β = –0.41, p = 0.003) and also after adjustment for age, sex and TKV (St. β = –0.23, p = 0.05). Conclusions: These data suggest that neither urine nor plas- ma osmolality are valid measures to identify ADPKD patients that may benefit from increasing water intake. Copeptin ap- pears a better alternative for this purpose. © 2015 S. Karger AG, Basel Introduction The antidiuretic hormone arginine vasopressin (AVP) is an essential hormone for osmoregulation. When plas- ma osmolality increases, AVP is secreted by the pituitary gland, subsequently activating the V2 receptors of renal collecting duct cells [1]; this results in the translocation of aquaporin 2 to the luminal surface of these cells, making them permeable for water [2]. Key Words Autosomal dominant polycystic kidney disease · Urine osmolality · Plasma osmolality · Vasopressin Abstract Background: Vasopressin plays an essential role in osmo- regulation, but has deleterious effects in patients with ADPKD. Increased water intake to suppress vasopressin ac- tivity has been suggested as a potential renoprotective strat- egy. This study investigated whether urine and plasma os- molality can be used as reflection of vasopressin activity in ADPKD patients. Methods: We measured urine and plasma osmolality, plasma copeptin concentration, total kidney vol- ume (TKV, by MRI) and GFR ( 125 I-iothalamate). In addition, change in estimated GFR (eGFR) during follow-up was as- sessed. Results: Ninety-four patients with ADPKD were in- cluded (56 males, age 40 ± 10, mGFR 77 ± 32 ml/min/1.73 m 2 , TKV 1.55 (0.99–2.40) l. Urine osmolality, plasma osmolali- ty and copeptin concentration were 420 ± 195, 289 ± 7 mOsmol/l and 7.3 (3.2–14.6) pmol/l, respectively. Plasma osmolality was associated with copeptin concentration (R = 0.54, p < 0.001), whereas urine osmolality was not (p = 0.4). In addition, urine osmolality was not associated with TKV Received: January 21, 2015 Accepted: April 3, 2015 Published online: April 25, 2015 Nephrolo gy American Journal of Ron T. Gansevoort Department of Nephrology, University Medical Center Groningen University of Groningen PO Box 30.001, NL–9700 RB Groningen (The Netherlands) E-Mail R.T.Gansevoort  @  umcg.nl © 2015 S. Karger AG, Basel 0250–8095/15/0413–0248$39.50/0 www.karger.com/ajn