E. Cranial diabetes insipidus – Explanation
A dramatic improvement is seen in the ability of the kidneys to concentrate urine following the
administration of DDAVP. This points towards a diagnosis of cranial diabetes insipidus
Water deprivation test
Method
- prevent patient drinking water
- ask patient to empty bladder
- hourly urine and plasma osmolalities
| Starting plasma osm | Final urine osm | Urine osm. post-DDAVP | |
| Normal | Normal | > 600 | > 600 |
| Psychogenic polydipsia | Low | > 400 | > 400 |
| Cranial DI | High | > 300 | > 600 |
| Nephrogenic DI | High | > 300 | > 300 |
