genitourinary disorders
Nephrogenic Diabetes Insipidus
Diagnosis of NDI
24-hour urine volume and osmolality Serum electrolytes Water deprivation test NDI is suspected in any patient with Polyuria . In infants, polyuria may be noticed by the caregivers; if not, the first manifestation may be dehydration. Initial testing includes 24-hour urine collection (without fluid restriction) for volume and osmolality, and serum electrolytes. Patients with NDI excrete>50 mL/kg of urine/day (polyuria). If urine osmolality is<300 mOsm/kg (300 mmol/L; known as water diuresis),argininevasopressindeficiency or NDI is likely. With NDI, urine osmolality is typically<200 mOsm/kg (200 mmol/L) despite clinical signs of hypovolemia (normally, urine osmolality is high in patients with hypovolemia). If osmolality is>300 mOsm/kg (300 mmol/L), solute diuresis is likely. Glucosuria and other causes of solute diuresis must be excluded. Serum sodium is mildly elevated (142 to 145 mEq/L, or 142 to 145 mmol/L) in patients with adequate free water intake but can be dramatically elevated in patients who do not have adequate access to free water. The diagnosis is confirmed by a water deprivation test, which assesses the maximum urine concentrating ability and response to exogenousvasopressin. During the test, urine volume and osmolality are measured hourly and serum osmolality is measured every 2 hours. After 3 to 6 hours of water deprivation, the maximal osmolality of urine in patients with NDI is abnormally low (<300 mOsm/kg, or 300 mmol/L). NDI can be distinguished fromargininevasopressindeficiency (lack ofvasopressin) by administering exogenousvasopressin(aqueousvasopressin5 units subcutaneously ordesmopressin10 mcg intranasally) and measuring urine osmolality. In patients withargininevasopressindeficiency, urine osmolality increases 50 to 100% over the 2 hours after administration of exogenousvasopressin(15 to 45% in partialargininevasopressindeficiency). Patients with NDI usually have only a minimal rise in urine osmolality (<50 mOsm/kg [50 mmol/L]; up to 45% in partial NDI). 