Pediatric Diabetes Insipidus Workup
- Author: Karl S Roth, MD; Chief Editor: Stephen Kemp, MD, PhD more...
Basic Laboratory Studies
In assessing patients with suspected diabetes insipidus (DI), 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.
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 central DI and nephrogenic DI on the basis of response to a vasopressin analogue.
The water deprivation test is performed as follows:
- Obtain baseline urine and blood for osmolality and electrolytes.
- 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.
- 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.
- If polyuria persists, administer intranasal desmopressin (see Medication), and replace urine output with fluids. After 4 hours (2 h 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 central DI and psychogenic DI but not in nephrogenic DI.
Magnetic Resonance Imaging
Cranial magnetic resonance imaging (MRI) can be used to exclude pituitary cysts, hypoplasia, and destruction secondary to mass lesions. Often, the bright spot on an MRI scan that is thought to represent vasopressin-secreting neurons in the posterior pituitary is absent in patients with central DI.
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