The thirst test (two-step test) is a diagnostic test designed to rule out diabetes insipidus. Diabetes insipidus is a congenital or acquired disease characterized by increased urine output (polyuria) and an increased feeling of thirst with increased drinking (polydipsia)
The procedure
Material needed
- Urine samples before, during and after the thirst test.
- Blood samples before, during and after the thirst test.
Preparation of the patient
- The patient may have a light breakfast with liquid on the morning of the trial day; coffee must not be drunk
- Immediately before the start of the test, the patient is weighed. Furthermore, a blood and urine sample (plasma/urine osmolarliät, plasma sodium concentration) must be taken.
- After that, do not drink for 12 h
- Every two hours, body weight, blood pressure, pulse, urine volume and urine osmolality – towards the end also serum sodium concentration and plasma osmolality – are measured
- At the end of the test, 20 μg of desmopressin (DDAVP) is administered intranasally
- In the next urine sample is measured urinosmolality
Interfering factors
- Drinking during the thirst test
Termination criteria
- Severe thirst
- Circulatory dysregulation (drop in blood pressure).
- Weight loss > 5% of initial weight
Contraindications
- Dehydration (lack of fluid)
Normal values
| Sample | Normal values |
| Plasma osmolality | <296 mosmol/kg bw |
| Urine osmolality | > 900 mosmol/kg bw |
| DDAVP | no increase |
Indication
- Suspected diabetes insipidus
Interpretation
Diabetes insipidus centralis
| Sample | Normal values |
| Plasma osmolality | > 296 mosmol/kg bw |
| Urine osmolality | Increase <10 mosmol/kg bw/h |
| DDAVP | Increase > 10 |
Diabetes insipidus renalis
| Sample | Normal values |
| Plasma osmolality | > 296 mosmol/kg bw |
| Urine osmolality | Increase <10 mosmol/kg bw/h |
| DDVAP | no increase |
