Laboratory Studies
In assessing patients with suspected arginine vasopressin (AVP) disorder (diabetes insipidus), the urine specific gravity of the first morning urine is helpful in assessing renal ability to concentrate urine. Dilute urine with a relatively high serum sodium and osmolarity effectively establishes the diagnosis. The serum sodium level may be as high as 170 mEq/L, while the serum osmolarity is greater than 300 mOsm/kg. Patients with prerenal azotemia present with severe dehydration.
In young infants, distinguishing between normal and pathologic inability to concentrate the urine may be difficult because infants generally exhibit a constitutional hyposthenuria.
An accurate 24-hour urine collection is important. The total urine output is high, and the number of osmoles excreted per day is small.
Serum potassium and calcium concentrations are important to exclude the possibility of polyuria secondary to hypokalemia or hypercalcemia, both of which interfere with renal concentrating mechanisms.
Copeptin concentration testing
In a prospective, multicenter, observational cohort study, a single baseline copeptin level greater than 21.4 pmol/L differentiated AVP resistance (AVP-R) (nephrogenic diabetes insipidus) from other etiologies with a 100% sensitivity and specificity and eliminated the need for water deprivation testing. [24] In another study, in patients undergoing pituitary procedures, low copeptin levels were indicative of postoperative AVP deficiency (AVP-D) (central diabetes insipidus), whereas high levels virtually excluded it. [26]
Water Deprivation Testing
The definitive diagnostic study is the water deprivation test, which can be used both to confirm the diagnosis and to distinguish between arginine vasopressin deficiency (AVP-D) (central diabetes insipidus) and AVP resistance (AVP-R) (nephrogenic diabetes insipidus) on the basis of response to a vasopressin analogue.
The water deprivation test is performed as follows:
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Obtain baseline urine and blood for osmolality and electrolytes.
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Deprive the patient of water after breakfast until significant dehydration occurs. Weigh the patient every 2 hours, and limit dehydration to 2-5% loss of body weight.
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Monitor urine specific gravity hourly; if the specific gravity is 1.014 or greater, terminate the test and obtain appropriate urine and blood specimens for osmolality. Limit water deprivation to 4 hours for infants and 7 hours for children.
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If polyuria persists, administer intranasal desmopressin, and replace urine output with fluids. After 4 hours (2 hr in infants), obtain urine and blood for osmolality.
The normal response to dehydration or desmopressin includes urine osmolality greater than 450 mOsm/kg, a urine-to-serum osmolality ratio of 1.5 or higher, and an increase in urine-to-serum osmolality of 1.0 or more from baseline. A normal response should be observed in AVP-D and psychogenic polydipsia but not in AVP-R.
Imaging Studies
Brain magnetic resonance imaging (MRI) at diagnosis can help determine the underlying cause of arginine vasopressin deficiency (AVP-D) (central diabetes insipidus). It can be used to exclude pituitary cysts, hypoplasia, and destruction secondary to mass lesions. [27] Diagnosis is confirmed via absence of the posterior pituitary bright spot (PPBS), which is nearly universal in patients with AVP-D, with the sensitivity of this MRI finding typically reported as between 90% and 100%. [19]
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Pediatric Arginine Vasopressin Disorders (Diabetes Insipidus). Carbs for Kids-Count Them In: The Constant Carbohydrates Diet.
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Pediatric Arginine Vasopressin Disorders (Diabetes Insipidus). Diabetes Sick Day Rules.
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Pediatric Arginine Vasopressin Disorders (Diabetes Insipidus). Taking Diabetes Back to School.
