Overnight water deprivation test

Document polyuria (urine volume >2.5L/day) and exclude glycosuria. If desired, creatinine excretion can be measured as an estimate of completeness of urine collections snbstances that inflnence ADH secretion should be avoided (e.g., nicotine, alcohol, and caffeine). If plasma osmolality is >295 mOsm/kg or if serum sodium concentration >145mmol/L, primary polydipsia is unlikely proceed with the overnight water deprivation test (Box 50-9) or the hypertonic saline infusion test (Box 50-10). 

Overnight water deprivation test (Box 50-9) If the ratio of urine to plasma osmolality is <1.5 at the end of the test, primary polydipsia is unlikeiy. Measure plasma and urine osmolalities and plasma ADH concentrations at the end of the test use these relationships to differentiate normal, nephrogenic, or hypothalamic diabetes insipidus, and psychogenic polydipsia. If urine osmoiahty is <400 mOsm/kg at the end of the test, give 5 U of aqueous vasopressin subcutaneously. If urine osmolality increases >10%, hypothalamic diabetes insipidus is probable if urine osmolality does not increase, nephrogenic diabetes insipidus is highly probable. 

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