Patient preparation:
The patient is fluid deprived for 16 hours (usually starting in the late evening) and no fluid intake is allowed during the test. Urine osmolality is monitored. Testing is done when the urine osmolality is relatively constant, with the difference between 2 specimens is < 30 mOsm/kg H2O. Water deprivation is hazardous in extra-renal uremia, renal failure with polyuria or poorly compensated renal damage
Hormone injection:
1) aqueous vasopressin, 5 U injected subcutaneously
2) aqueous pitressin, injected subcutaneously
3) DDAVP (a synthetic vasopressin analogue), 20 µg intranasally
4) DDAVP (a synthetic vasopressin analogue), 2 µg intravenously
Specimens collected:
1) serum osmolality before injection
2) urine osmolality before injection
3) serum osmolality 1 hour after injection
4) urine osmolality 1 hour after injection
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Serum osmolality at plateau (mOsm/kg H2O)
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Urine osmolality at plateau
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Urine osmolality after hormone injection
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normal person
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< 300
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> serum
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< 5% increase
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primary polydipsia
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< 300
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> serum
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< 5% increase [*]
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partial diabetes insipidus
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< 300
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> serum
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> 9% increase
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severe diabetes insipidus
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> 300
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< serum
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> 50% increase
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nephrogenic diabetes insipidus
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> 300
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< serum
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< 50% increase
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[*] If the ratio of urine-to-plasma osmolality is < 1.5 at the end of the test, primary polydipsia is unlikely.
Limitation:
1) Serum antidiuretic hormone level determinations are better for diagnosis.
2) The test is unreliable in pregnancy, with liver disease, with low salt or low protein diet, and in the presence of any severe fluid and/or electrolyte disorder.
3) Patients with psychogenic polydipsia will show a normal response, but water deprivation may need to be prolonged.
