WATER DEPRIVATION TEST

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Specimen: Urine; 2 mL blood in a lithium heparin tube collected at intervals during the test.

Method: 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 hour) or the patient becomes dehydrated (body weight decrease >3%). Desmopressin is then given and a further urine specimen is taken. See OSMOLALITY - serum and OSMOLALITY - urine .

Reference Interval: Urine osmolality >2 x plasma osmolality with <9% increase after DDAVP.

Application: Diagnosis of central or nephrogenic diabetes insipidus.

Interpretation: Complete central diabetes insipidus: urine osmolality 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: urine osmolality <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.

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