Urine and plasma osmolality in patients with autosomal dominant polycystic kidney disease: reliable indicators of vasopressin activity and disease prognosis?

BACKGROUND: Vasopressin plays an essential role in osmoregulation, but has deleterious effects in patients with ADPKD. Increased water intake to suppress vasopressin activity has been suggested as a potential renoprotective strategy. This study investigated whether urine and plasma osmolality can be used as reflection of vasopressin activity in ADPKD patients.
METHODS: We measured urine and plasma osmolality, plasma copeptin concentration, total kidney volume (TKV, by MRI) and GFR ((125)I-iothalamate). In addition, change in estimated GFR (eGFR) during follow-up was assessed.
RESULTS: Ninety-four patients with ADPKD were included (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 osmolality 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 (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 copeptin 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 plasma osmolality are valid measures to identify ADPKD patients that may benefit from increasing water intake. Copeptin appears a better alternative for this purpose.