A water deprivation test, used to diagnose diabetes insipidus, involves measuring urine and blood osmolality after fluid restriction and, optionally, after administering desmopressin (synthetic ADH). 

Here’s a breakdown of the test and its interpretation:

• Purpose:The test assesses the kidneys’ ability to concentrate urine in response to reduced fluid intake, which is crucial for diagnosing diabetes insipidus (DI), a condition characterized by excessive thirst and urination.
• Procedure:
  • Patients are instructed to abstain from fluids for a period (typically 8 hours or until a certain weight loss or serum osmolality is reached).
  • Urine and blood samples are collected periodically to measure osmolality (concentration of particles in the fluid).
  • After the initial water deprivation period, desmopressin (a synthetic ADH) may be administered, and further urine and blood samples are collected to assess the kidneys’ response.
• Interpreting the Results:
  • Normal Response: In healthy individuals, urine osmolality increases significantly during water deprivation, indicating that the kidneys are effectively concentrating urine.
  • Diabetes Insipidus (DI):
    • Central DI: Urine osmolality remains low (below 300 mOsm/kg) even after water deprivation, but increases significantly after desmopressin administration (indicating the kidneys can respond to ADH).
    • Nephrogenic DI: Urine osmolality remains low even after water deprivation and does not increase significantly after desmopressin administration (indicating the kidneys cannot respond to ADH).
  • Primary Polydipsia: Urine osmolality increases during water deprivation, but not as much as in a normal response, and may not increase significantly after desmopressin administration.
• Other Considerations:
  • The test should be terminated if the patient loses more than 5% of their body weight or if serum osmolality becomes too high.
  • The test may be inconclusive in patients with partial DI or chronic primary polydipsia.
  • The test results should be interpreted in conjunction with other clinical findings.