Should be carried out in conditions of strict monitoring and in centres with experience with this test:
Allow fluids overnight. If primary polydipsia is suspected consider overnight fluid deprivation to avoid over hydration
Commence fluid deprivation at 8 a.m.
Serum osmolality, serum Na+, urine osmolalitiy. Each time urine sample voided
Duration of water deprivation is seldom longer than 8–12hr in children and 6–8hr in young infants. In any case, the water deprivation is terminated if there is either:
urine osmolality concentrated: ≥800mOsm/kg or
thirst becomes intolerable or
5% dehydration (5% weight loss) or
Serum osmolality: ≥300mOsm/L.
In those with inadequate urinary concentration, desmopressin is administered: DDAVP 0.1mg/kg to maximum of 4mg IM
Interpretation of results: see table that follows
| . | Urine osmolality (mOsm/kg) . | |
|---|---|---|
| . | After fluid deprivation . | After DDAVP . |
Cranial DI (CDI) | <300 | >800 |
Nephrogenic | <300 | >300 |
Primary polydipsia | >800 | >800 |
Partial CDI/polydipsia | >00–800 | <800 |
| . | Urine osmolality (mOsm/kg) . | |
|---|---|---|
| . | After fluid deprivation . | After DDAVP . |
Cranial DI (CDI) | <300 | >800 |
Nephrogenic | <300 | >300 |
Primary polydipsia | >800 | >800 |
Partial CDI/polydipsia | >00–800 | <800 |
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