Diagnosis and Differentiation of Diabetes Insipidus

Initial Biochemical Screening

The initial laboratory workup for suspected diabetes insipidus requires simultaneous measurement of serum sodium, serum osmolality, and urine osmolality, which confirms the diagnosis when urine osmolality is <200 mOsm/kg with high-normal or elevated serum sodium, according to the American College of Physicians 1, 2
Serum sodium levels should be checked for high-normal or elevated levels (>145 mmol/L indicates hypernatremia), as recommended by the American College of Physicians 1, 2
Serum osmolality is typically elevated or high-normal in diabetes insipidus, as stated by the National Institute of Diabetes and Digestive and Kidney Diseases 1, 2
Urine osmolality is a hallmark finding, with inappropriately dilute urine (<200 mOsm/kg) despite serum hyperosmolality, according to the American Diabetes Association 1, 2
Polyuria is defined as >3 liters per 24 hours in adults or >2.5 L/day, as defined by the European Association for the Study of Diabetes 2

Plasma copeptin measurement is the primary differentiating test, which has largely replaced direct AVP measurement due to superior stability and ease of measurement, as recommended by the Endocrine Society 1, 2
Baseline copeptin >21.4 pmol/L is diagnostic for nephrogenic diabetes insipidus with high precision, according to the International Society of Nephrology 1, 2
Baseline copeptin <21.4 pmol/L indicates either central diabetes insipidus or primary polydipsia, requiring additional stimulation testing, as stated by the American College of Endocrinology 1, 2
The hypertonic saline infusion test is used to differentiate central DI from primary polydipsia with superior accuracy compared to water deprivation, as recommended by the National Institute of Diabetes and Digestive and Kidney Diseases 1

For confirmed or suspected nephrogenic diabetes insipidus, genetic testing with a multigene panel including AVPR2, AQP2, and AVP genes is strongly recommended, even in adults and females, as recommended by the American Society of Human Genetics 1, 2
Genetic testing should be performed in accredited diagnostic laboratories and includes copy number variant analysis, as stated by the College of American Pathologists 1
Genetic testing identifies pathogenic variants in 90-95% of cases and is essential for family planning, genetic counseling, and presymptomatic testing of at-risk family members, according to the National Society of Genetic Counselors 1

Serum electrolytes, including sodium, potassium, chloride, and bicarbonate, should be measured as part of the baseline metabolic panel, as recommended by the American College of Physicians 2
Serum creatinine and uric acid should be measured to assess kidney function and monitor for complications, as stated by the National Kidney Foundation 2
Serum sodium should be monitored within 7 days and at 1 month after starting treatment, then periodically, to prevent hyponatremia, a main complication of desmopressin therapy, as recommended by the Endocrine Society 2

MRI with dedicated pituitary/sella sequences is essential when central diabetes insipidus is suspected, as it identifies structural causes including tumors, infiltrative diseases, and post-surgical changes, as recommended by the American College of Radiology 2