Management of Polyuria and Polydipsia with Increased Urine Osmolarity After Water Deprivation
The correct answer is B. Water restriction is the appropriate management for this patient, as increased urine osmolarity following water deprivation indicates primary polydipsia, NOT diabetes insipidus. 1
Critical Diagnostic Interpretation
The key to this question is understanding what "increased urine osmolarity" after water deprivation actually means:
In diabetes insipidus (either central or nephrogenic), urine osmolality remains inappropriately dilute (<200 mOsm/kg) despite water deprivation because the kidneys cannot concentrate urine due to either lack of ADH or renal resistance to ADH 1, 2
When urine osmolality increases appropriately during water deprivation (typically >300-800 mOsm/kg), this demonstrates intact ADH secretion and normal kidney response, effectively ruling out diabetes insipidus 3, 4
This pattern is pathognomonic for primary polydipsia, where excessive fluid intake causes polyuria but the kidneys retain normal concentrating ability 1, 3
Why Desmopressin is WRONG in This Case
Desmopressin is only indicated for central diabetes insipidus, where there is deficient ADH secretion 1, 2, 5. Administering desmopressin to a patient with primary polydipsia who continues excessive fluid intake can cause life-threatening dilutional hyponatremia 1, 6.
The FDA label explicitly states desmopressin is "ineffective for the treatment of nephrogenic diabetes insipidus" and is only indicated when diagnosis is established showing inability to concentrate urine 5.
Correct Management Approach
For primary polydipsia (confirmed by appropriate urine concentration during water deprivation):
Behavioral modification with fluid restriction is the cornerstone of treatment - patients must reduce their excessive fluid intake to normal physiologic amounts 1
Unlike diabetes insipidus patients who require free access to fluids to prevent hypernatremic dehydration, primary polydipsia patients need the opposite approach 1, 2
Psychiatric evaluation may be warranted, as primary polydipsia can be associated with psychiatric disorders or compulsive behaviors 1
Common Diagnostic Pitfall
Do not confuse the water deprivation test interpretation:
Diabetes insipidus: Urine osmolality STAYS LOW (<200-300 mOsm/kg) despite dehydration, then may increase >50% with desmopressin (central DI) or shows no response (nephrogenic DI) 1, 7, 4
Primary polydipsia: Urine osmolality INCREASES appropriately (>300-800 mOsm/kg) during water deprivation alone, with no need for desmopressin 3, 4
The question specifically states urine osmolarity "increased" during water deprivation, which definitively excludes diabetes insipidus and makes desmopressin both unnecessary and potentially dangerous 1, 3.