Investigations

General screening:

• blood glucose, plasma calcium and potassium – hypercalcemia and hypokalemia can cause nephrogenic diabetes insipidus
• 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

References:

1. Levy M, Prentice M, Wass J. Diabetes insipidus. BMJ. 2019 Feb 28;364:l321. doi: 10.1136/bmj.l321.