An 18·year-old man presents to his GP with th1rst and polyuria. Some 6 months previously he had a significant head injury as the result of a road traff iC accident. He is referred to the local Endocrine Clinic.
Which of the following results would be the most useful in conforming a diagnosis of diabetes insipidus after a water deprivation test (without additional desmopressin)?
A Plasma sodium of 126 mmol/1
B Plasma sodium of 150 mmol/1
c Plasma osmolality of 335 mosmol/kg and urine osmolality of 700 mosmol/kg
D Plasma osmolality of 280 mosmol/kg and urine osmolality of 700 mosmol/kg
Explanation
Diabetes insipidus
Diabetes insipidus (DI) can be cranial diabetes insipidus, whh complete or partial deficiency of antidiuretic hormone (ADH, vasopressin), or renal diabetes insipidus due to renal tubular defects affecting the action of AOH on water reabsorption from the renal tubules.
Biochemical abnormalities
The plasma sodium can be normal or elevated, depending on whether the patient can continue to drink freely and so prevent dehydration. A low plasma sodium concentration is associated with SIADH (syndrome of inappropriate ADH secretion). The diagnostic test to conf1rm 01 is a water deprivation test The inability to concentrate the urine during the test results in the plasma osmolality rising and the urine osmolality remaining dilute. Normal plasma osmolality is 285305 mosmol/kg. The higher the urine osmolality the more concentrated it is