Water deprivation test

- Patients are deprived of fluid for 8 hours or until 5% of body weight is loss
- patient should be weighed hourly
- plasma osmolality is measured 4 hourly
- urine volume and osmolality 2 hourly
- then give 2microgram desmopressin IM and check urine volume & osmolality and plasma osmolality over next 4 hours
- Interpretation
 ~ if serum osmolality > 305 mOsmol/kg = diabetes insipidus
 ~ urine osmolality < 300 mOsmol/kg after fluid deprivation and rising to above 800mOsmol/kg after desmopressin = cranial diabetes insipidus
 ~ urine osmolality < 300 mOsmol/kg after fluid deprivation and remains < 300mOsmol/kg after desmopressin = nephrogenic diabetes insipidus
 ~ urine osmolality > 800 mOsmol/kg after fluid deprivation without desmopressin = primary polydipsia
 ~ urine osmolality between 300 and 800 mOsmol/kg = partial DI/polydipsia