Redefining Diagnostic Cut-Offs for the Indirect Water Deprivation Test

Area of Science:

• Endocrinology
• Nephrology
• Clinical Diagnostics

Background:

• Incorrect diagnosis of arginine vasopressin deficiency (AVP-D) and resistance (AVP-R) can lead to dangerous desmopressin use in healthy individuals.
• The water deprivation test is crucial for diagnosing polydipsia polyuria syndrome (PPS).
• Current reference ranges for diagnostic tests may require re-evaluation for improved accuracy.

Purpose of the Study:

• To evaluate the diagnostic effectiveness of current reference ranges used in the water deprivation test.
• To determine the optimal cut-off values for serum sodium and urine osmolality in differentiating AVP-D, AVP-R, and primary polydipsia (PP).

Main Methods:

• Retrospective analysis of 135 patients undergoing water deprivation tests (August 2014 - August 2023).
• Review of patient diagnoses and determination of variability and ROC curves for serum osmolality, serum sodium, and urine osmolality.
• Analysis of post-desmopressin urine osmolality and clinical monitoring in equivocal cases.

Main Results:

• Serum sodium showed significantly lower variability (0.72%) compared to serum osmolality (1.16%).
• A serum sodium cut-off of ≥ 148 mmol/L demonstrated 100% specificity in excluding PP.
• Combined use of serum sodium (≥ 148 mmol/L) and urine osmolality cut-offs (> 630 mOsm/kg for PP, < 383 mOsm/kg for AVP-D/AVP-R) achieved high diagnostic accuracy, with equivocal cases resolved with 100% sensitivity and specificity.

Conclusions:

• A serum sodium cut-off of ≥ 148 mmol/L, combined with urine osmolality, provides superior diagnostic accuracy for differentiating AVP-D, AVP-R, and PP.
• Serum sodium is a more reliable marker than serum osmolality in PPS investigations due to lower biological and analytical variability.
• Optimized diagnostic criteria can prevent misdiagnosis and inappropriate treatment in patients with PPS.