A 45 year old man is being investigated in the endocrinology clinic after presenting with polyuria, estimated to be over 3 litres of urine per day. Plasma osmolality is found to be high and urine osmolality is low, and his Haemoglobin A1c (HbA1c) is normal. His past medical history includes a recent head injury.
What is the most appropriate test to diagnose the likely condition?
