Hypertonic saline infusion
This test is potentially dangerous and must be undertaken with great care. Patients unable to conserve water may rapidly become critically hypertonic during this test.
To make a clear diagnosis of cranial diabetes insipidus in subjects with polyuria and normal plasma osmolality.
Patients with epilepsy, cerebral or cardiovascular disease.
The test is designed to stress the integrity of the renal-ADH axis. The infusion of hypertonic saline raises plasma osmolality and ensures maximal stimulation of ADH secretion. The failure of maximal renal concentration of urine does not help differentiate which organ is performing suboptimally. The diagnosis can be seen by comparing the response of ADH to plasma osmolality using the Newcastle chart (Prof PH Baylis)
There is a serious risk of dehydration in patients with DI
The hypertonic saline may induce thrombophlebitis at the site of infusion.
Fast patient from midnight before the day of the test. Allow water only to be drunk until time of test (maximum volume 500 mL). No tea, coffee, alcohol or smoking after midnight. Patients should continue on any replacement therapy.
- Infusion pump and IV sets
- 5% saline (500-1000 mL)
- Accurate scales for weighing the patient
- Volumetric flasks (200 mL) for measuring urine volumes
- 8 lithium heparin blood sample tubes and syringes cooled on ice
- 8 urine sample bottles
** Discuss with the laboratory on the day before the test **
1. Patient instructed to empty bladder. Measure urine volume and osmolality
2. Weigh patient
3. Patient to lie supine
4. Insert cannula into antecubital veins of both arms. Allow patient to rest for 30 min.
5. Take blood into chilled Li Heparin tube.
6. Repeat blood sample after 15 min
7. Begin infusion of 5% saline at 0.04 mL/kg/min for 2 hours into non-blood sampling arm
8. Take blood samples at 30 min intervals.
9. Measure volume and osmolality on all urine passed.
Note time at which thirst is noted - if patient very thirsty during test, give ice chips
10. Take final blood sample 15 min after completion of infusion.
11. Record blood pressure, urine volume, blood sampling, patients comments
12. Allow patient to drink after test. Avoid ingestion of large fluid volumes.
Patients with primary polydipsia or nephrogenic diabetes insipidus have normal AVP release in response to the hyperosmolar state induced by this procedure. Patients with cranial diabetes insipidus have little or no rise in AVP.
- Baylis PH. Robertson GL. Plasma vasopressin response to hypertonic saline infusion to assess posterior pituitary function. J Roy Soc Med (Lond) 1980;73:255-60.
- Baylis PH, Phillips EMG. The endocrine investigation of disorders of sodium and water homeostasis. J Int Fed Clin Chem 1994;6:158-164.