An 8-year-old boy is brought to the office due to a 2-month history of excessive urination. The parents report that the boy is sometimes irritable and restless and has been drinking excessive amounts of water. He has had normal growth with achievement of all expected milestones and takes no medications. The patient's maternal grandfather tends to drink excess amounts of water. Vital signs reveal tachycardia. On physical examination, mucous membranes are dry. Neurologic examination shows no focal deficits. Laboratory results on a random blood sample are as follows: Urine osmolality is 120 mOsm/kg H₂O. Which of the following is the most appropriate next step in management?

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Explanation: 11ec2b69_beb0_0619_8f63_3f7f7088593d_MD0005_00 This patient has polydipsia, polyuria, and dilute urine (urine osmolality <300 mOsm/kg H₂O, urine specific gravity <1.006). The differential diagnosis includes diabetes insipidus and primary polydipsia; these can be differentiated using the water deprivation test. Water deprivation (eg, overnight fast) elevates serum sodium and serum osmolality; therefore, maximal secretion of antidiuretic hormone (ADH) (with resulting increased renal water reabsorption) and highly concentrated urine (eg, urine osmolality >600 mOsm/kg H₂O) are normally expected following water deprivation. In diabetes insipidus and primary polydipsia, water deprivation produces the following results: In diabetes insipidus, renal concentrating ability is impaired due to renal ADH resistance (nephrogenic diabetes insipidus) or deficient ADH production (central diabetes insipidus). Therefore, urine remains dilute (urine osmolality <300 mOsm/kg H₂O) following water deprivation. In young children, hereditary nephrogenic diabetes insipidus (eg, family history of polydypsia/polyuria) is the most common cause. In primary polydipsia, excessive water intake suppresses ADH, and polyuria is an appropriate response to excess water load. Following water deprivation, ADH increases and urine becomes concentrated. Primary polydipsia is associated with psychiatric or CNS disorders. The water deprivation test is performed under hospital supervision in older children and should not be used in very young children due to the risk of severe hypernatremia. An alternate test is the administration of desmopressin (an ADH analogue) to assess for renal concentrating ability. (Choice A) Lung malignancy requiring a chest x-ray can be associated with syndrome of inappropriate ADH secretion, a condition of impaired water excretion typically associated with hyponatremia and concentrated urine. (Choice B) A low fractional excretion of sodium (eg, <1%) may suggest a prerenal cause of acute kidney injury (eg, dehydration) but does not assess renal urinary concentrating capacity or narrow the diagnosis. (Choices C and E) Once the diagnosis of diabetes insipidus is established, either MRI or renal ultrasound may be appropriate for evaluating possible underlying causes (eg, tumor causing central DI, intrinsic kidney damage causing nephrogenic diabetes insipidus). However, water deprivation is a more appropriate initial step for narrowing the differential diagnosis. (Choice D) Diabetes mellitus can present with polyuria and polydipsia but is typically associated with osmotic diuresis (ie, from glucosuria) and concentrated, rather than dilute, urine. Educational objective: The differential diagnosis for polyuria and dilute urine includes central or nephrogenic diabetes insipidus (DI) or primary polydipsia. The water deprivation test can differentiate between DI and primary polydipsia. Following water deprivation, urine osmolality remains unchanged in DI but increases in primary polydipsia.

An 8-Year-Old Boy Is Brought to the Office Due to a 2-Month