Water deprivation test

Specimen:

Urine; 5 mL blood in a lithium heparin tube collected at intervals during the test.

Protocol:

Medical supervision must be provided during the test, which must be performed in a hospital setting.

The patient is allowed no oral or parenteral intake.

Plasma and urine specimens are collected hourly until osmolality reaches a plateau (< 30 mmol/kg increase over 1 h) or the patient becomes dehydrated (body weight decrease > 3 %). Desmopressin is then given and a further urine specimen is taken.

See Osmolality and Osmolality urine.

Reference Interval:

Osmolality urine > 2 x plasma Osmolality with < 9 % increase after DDAVP.

Application:

Diagnosis of central or nephrogenic Diabetes insipidus.

Interpretation:

Complete central Diabetes insipidus: Osmolality urine remains lower than plasma Osmolality with > 50 % increase after DDAVP.

Partial central Diabetes insipidus: urine shows 10-50 % increase in Osmolality after DDAVP.

Nephrogenic Diabetes insipidus: Osmolality urine < 2 x plasma Osmolality with < 9 % increase after DDAVP.

Psychogenic polydipsia may resemble nephrogenic Diabetes insipidus, but the patient is usually hypo-osmolar and hyponatraemic.

Reference:

Miller M et al. Ann Int Med 1970; 73: 721-729. C

Heetham T and Baylis PH. Paediatr Drugs 2002; 4(12): 785-796.