investigations
Last reviewed 06/2023
General screening:
- blood glucose, plasma calcium and potassium
- serum sodium - may be raised
- serum and urine osmolarity - plasma osmolality should be high and urine osmolality should be low. In cases of psychogenic polydipsia then generally the plasma osmolality is low
- confirm existence of polyuria - more than 3.0 l urine in 24 hours
More specialist investigations:
- fluid deprivation test with addition of exogenous vasopressin - usually, desmopressin
- measurement of plasma ADH levels in response to hypertonic saline infusion in difficult cases
- therapeutic trial of vasopressin - requires specialist supervision and indicated only if plasma ADH measurements are not available and the diagnosis is uncertain - 10 mcg per day given intranasally - cranial diabetes insipidus patients improve, nephrogenic diabetics are unchanged, primary polydipsia patients develop progressive hyponatraemia