Laboratory Test: The patient was then subjectedto a 2-hour water deprivation test followed by another bloodchemistry profile and urinalysis. Of note, serum osmolalityincreased to 329mOsm/kg while serum ADH, urine specific gravity andurine osmolality remained unchanged.
Questions to Consider:
- Why did the patient’s serum osmolality increase?
Follow-up and Diagnosis: The patient was theninjected with a drug called DDAVP (desmopressin) which mimics theactions of ADH. One hour after the injection, serum osmolalitydecreased to 292mOsm/kg and urine osmolality increased to480mOsm/kg. Based on the patient’s medical history, the resultsfrom lab tests and the, you make the differential diagnosis ofidiopathic central diabetes insipidus.
Questions to Consider:
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- Why do you suspect serum osmolality decreased? Why do yoususpect urine osmolality increased? Based on these results, what ishappening to water?
- What do these results suggest is the underlying cause ofdiabetes insipidus?
- How does the diuresis experienced in diabetes insipidus differmechanistically from that experienced by a patient with diabetesmellitus?
