Poster Presentation ESA-SRB 2023 in conjunction with ENSA

Diagnostic accuracy of the water deprivation test combined with fasting copeptin in the polyuria-polydipsia syndrome (#236)

Claudia Ashkar 1 , Maresa Derbyshire 1 , Balasubramanian Krishnamurthy 1 2 , Richard MacIsaac 1 3 , Nirupa Sachithanandan 1
  1. Endocrinology and Diabetes, St Vincent's Hospital, Melbourne, VIC, Australia
  2. St Vincent's Institute of Medical Research, Melbourne, VIC, Australia
  3. University of Melbourne, Melbourne, VIC, Australia

Aims: The water deprivation test (WDT) is the gold standard investigation to distinguish the diagnosis for the polyuria-polydipsia syndrome. Fasted plasma copeptin has emerged as a useful adjunct to distinguish the diagnosis. The aim of this study was to assess the diagnostic accuracy of the WDT and the utility of 8hr fasted copeptin. 

Methods: This was a retrospective cohort study of 63 outpatient WDTs completed between 2012-2022 in a tertiary referral centre. A standardized WDT was undertaken. In 12 cases, plasma copeptin was measured at 8 a.m. following 8hr overnight fast. Diagnosis was based on standard biochemical diagnostic criteria and, where results were equivocal, independent review of clinical features and response to treatment was undertaken by two Endocrinologists.

Results: Sixty-one patients underwent 63 WDT’s and 37 (60.7%) were diagnosed with primary polydipsia, 17 (27.9%) were diagnosed with partial central diabetes insipidus (DI), 6 (9.8%) with complete central DI and 1 (1.6%) was inconclusive. The diagnosis from WDT alone was concordant with predefined biochemical diagnostic criteria in 53 (84.1%) tests. Where a fasted copeptin was measured, the corresponding serum sodium was >147 mmol/L in one patient (1/12, 8.3%) and serum osmolality was >300 mOsm/kg in 6 patients (6/12, 50%). In patients with primary polydipsia the median 8hr fasted copeptin (3.3 pmol/L, IQR 3.1-3.9) was significantly higher compared to patients with partial central DI (2.25 pmol/L, IQR 2.1-4.5, p=0.003). A fasted copeptin (pmol/L) to serum sodium (mmol/L) ratio >0.02 demonstrated a sensitivity of 77.8%, specificity of 75% and positive predictive value of 87.5% in diagnosing primary polydipsia.

Conclusion: WDT identifies a clear diagnosis in the majority of presentations with polyuria-polydipsia syndrome. The plasma copeptin following 8hr overnight fast did not improve the diagnostic accuracy of WDT, as most patients had not yet reached the osmotic threshold to stimulate copeptin release.

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