Expansion
Deprive water, measure urine osmolality, then give desmopressin
Expansion
| Condition | After deprivation | After desmopressin |
|---|---|---|
| Normal | Urine concentrates (over 600 mOsm/kg) | No further change |
| Primary polydipsia | Concentrates, though sometimes submaximally | No further change |
| Cranial diabetes insipidus | Stays dilute | Concentrates (rises over 50 per cent) |
| Nephrogenic diabetes insipidus | Stays dilute | No response |
The test must be supervised, with hourly weights and plasma osmolality, and stopped if weight falls by more than 3 per cent or plasma osmolality exceeds 300 mOsm/kg, since a patient with true diabetes insipidus can become dangerously dehydrated.
Pitfalls
- Long-standing primary polydipsia washes out the medullary gradient, so concentrating ability is blunted and the result resembles partial diabetes insipidus
- Partial forms of both types give intermediate results
Measurement of copeptin, a stable cleavage product co-secreted with vasopressin, is increasingly replacing this test, being simpler and more discriminating, particularly after hypertonic saline stimulation.