Question 1 [15 marks] A 42 year old male with a oligoastrocytoma is on chemotherapy treatment with temozolomide (TMZ). Two months after initiation of TMZ the patient presents with polydipsia, polyuria and nocturia. Laboratory testing revealed hypernatremia and hyperosmolarity. An overnight water deprivation tests was attempted, but was not completed. Anterior pituitary function was assessed and shown to be normal. The patient was empirically placed on 1-desamino-8-D-arginine vasopressin acetate (DDAVP) which resolved his symptoms. Initial laboratory results Test Result Reference Interval Plasma Na 151 mmol/L 135-145 mmol/L Plasma Osmolality 319 mOsm/Kg 284-295 mOsm/Kg Urine Osmolality 222 mOsm/kg Laboratory results post-DDAVP treatment: Test Result Reference Interval Plasma Na 143 mmol/L 135-145 mmol/L Plasma Osmolality 294 mOsm/kg 284-295 mOsm/Kg Urine Osmolality 602 mOsm/kg a) Identify the clinical condition; please be as specific as possible. [2 marks] Describe a water deprivation tests. How is it performed? How is it interpreted? What results would you expect in this patient? [10 marks]. b) Why is the patient initially hypernatremic? Explain the hormonal mechanism that causes hypernatremia in this disorder. [3 marks] Question 2 [5 marks] Describe how cut-points are chosen for diagnostic tests. How can this approach also be used to compare the performance of different tests and determine optimal cut-point(s)? [5 marks]