General: Nephrogenic diabetes insipidus (NDI) should be considered in people with polyuria (excessive urine production) and/or polydipsia (excessive thirst). Blood and urine tests are performed to test for abnormal levels of water, sodium, and arginine vasopressin
Water deprivation test: Patients with NDI tend to produce urine that has a low concentration despite being deprived of water. On administration of AVP, patients with NDI will show little or no increase in urine concentration. During the water deprivation test, the patient is carefully monitored to make sure his or her body weight and blood plasma concentration stay within a safe range. The patient goes without water for less than six hours while the blood plasma concentrations and urine volume are measured. If the patient has NDI, he or she will be resistant to the antidiuretic action of AVP. Immediately following the water deprivation period, a patient with NDI will still have concentrated blood plasma and dilute urine despite the fact that he or she will be fairly dehydrated at that point. At the end of the water fast, the patient's response to an infusion of DDAVP is measured. If the patient does not respond to DDAVP, he or she may have NDI. If the patient shows highly concentrated urine in response to DDAVP, he or she may have a different form of diabetes insipidus called pituitary diabetes insipidus, which is also called central or neurogenic diabetes insipidus. Following DDAVP administration, a patient with NDI caused by a mutant AQP2 gene will display an increased heart rate and blood pressure, whereas the patient who has NDI caused by a mutated V2R gene will not.
Ultrasound: An ultrasound may be performed to examine for swelling of the kidney, the urinary tract, and the bladder to establish the extent of disease in an individual diagnosed with NDI. Some degree of urinary tract swelling may be seen on ultrasound examination even in infants.
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