Q. A 35-year-old female presents to the emergency department with complaints of headache, confusion, and excessive thirst for the past three days. She has a history of small cell lung cancer, for which she has been undergoing chemotherapy for the past six months. On physical examination, the patient appears confused and disoriented. Her blood pressure is 106/62 mm Hg, heart rate 120/min, respiratory rate 18/min, and her temperature is 37 degrees Celsius. She has decreased skin turgor and sunken eyes. Laboratory tests reveal a sodium level of 121 mEq/L and a urine osmolality of 460 mOsm/kg. Her urine sodium is 60 mEq/L, and she has a low urine output. These findings, in conjunction with her symptoms and medical history, are suggestive of the syndrome of inappropriate antidiuretic hormone secretion. The patient is started on fluid restriction and a normal saline infusion is initiated to slowly raise her sodium levels. A chest X-ray shows no evidence of lung infiltrate or pleural effusion, and a computed tomography scan of her brain is normal. The patient is admitted to the hospital for further management and her chemotherapy is held temporarily. Based on widely accepted clinical criterion for diagnosing this condition, which of the following is NOT required to make the diagnosis?