OPEN Endocrine connections | 26 Feb 2015
M de Fost, S Oussaada, E Endert, G Linthorst, M Serlie, M Soeters, JH DeVries, P Bisschop and E Fliers
Background: The water deprivation test is the gold standard test to differentiate central or nephrogenic diabetes insipidus (DI) from primary polydipsia (PP) in patients with polyuria and polydipsia. Few studies have addressed the diagnostic performance of this test. Methods: The aim of this retrospective cohort study was to evaluate the diagnostic performance of the standard water deprivation test, including plasma arginine vasopressin (AVP) measurements, in 40 consecutive patients with polyuria. We compared initial test results to the final clinical diagnosis, i.e., no DI, central DI or nephrogenic DI. Median length of follow up was 8 years. In a subset of 10 patients, the novel marker copeptin (CP) was measured in plasma. Results: Using the final diagnosis as gold standard, a threshold for urine osmolality of > 800 mOsmol/kg after water deprivation yielded a sensitivity and specificity of 96% and 100% for diagnosing PP. Sensitivity increased to 100% if the cut off value for urine osmolalilty was set at 680 mOsmol/kg. Plasma AVP levels did not differ between patient groups and did not differentiate between central DI, nephrogenic DI or PP. In all three patients with central DI, plasma CP was <2.5 pmol/L with plasma osmolality > 290 mOsmol/kg, and >2.5 pmol/L in patients without DI. Conclusions: The optimal cut-off value for differentiating PP from DI during a water deprivation test was urine osmolality > 680 mOsmol/kg. Differentiating between central and nephrogenic DI should be based on clinical judgement since AVP levels did not discriminate.
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