IMPE Abstracts

Background: Polyuria and polydipsia can manifest at any age and the determination of a specific diagnosis can be more challenging at a younger age. Once osmotic diuresis has been ruled out, polyuria can be explained by one of three conditions: insufficient production of antidiuretic hormone (Central diabetes insipidus –CDI), decreased renal sensitivity to antidiuretic hormone (nephrogenic diabetes insipidus – NDI) or excessive fluid intake (Primary polydipsia-PP). Morning assessment can be helpful as random morning plasma osmolality after discontinuing water intake at midnight >295 mOsm/kg and/or serum sodium >143 mmol/L with urine osmolality <300 mOsm/kg or urine/plasma osmolality ratio <1 are diagnostic of CDI. A random morning urine osmolality >700-800 mOsm/kg rules out DI. Most patients fall into a gray area where the next step of evaluation is a water deprivation test and vasopressin (DDAVP) trial which is usually cumbersome and has limited sensitivity and specificity. Recently, copeptin, the C-terminal segment of the AVP precursor peptide, became available as an attractive new surrogate marker for the diagnosis of DI. Arginine can stimulate the posterior pituitary and might therefore provide a simple and alternative diagnostic test in the differential diagnosis of DI. In adults, a copeptin cutoff of 3.8 pmol/L after arginine infusion had an accuracy of 93% in differentiating between CDI and PP, with a sensitivity of 93% and a specificity of 92%.

Methods: 3 y.o. female, healthy was referred to our clinic due to polyuria and polydipsia of 70-90 oz. in 24 hours (h) and wet diapers every 2-3 h day and night. NDI was ruled by nephrology with a random copeptin 2.6 pmol/L (Normal 2-26). She came to our infusion center at 8 am after an overnight fast of 8 h and fluid restriction for 2 h. She underwent an arginine stimulation test (dose: 0.5 g/kg). At baseline and 30, 60, and 120 min after the start of arginine infusion, blood pressure and pulse rate were monitored and blood was drawn for copeptin measurement.

Results: Her copeptin results showed a normal curve with a baseline value of 2.9 pmol/L, 30 min 4.8 pmol/L, 60 min 5.5 pmol/L, 120 5.9 pmol/L. Blood pressure and heart rate were normal

Conclusions: Arginine-stimulated copeptin measurement is a simple, novel, and safe diagnostic approach to diabetes insipidus in clinical practice. More data on children is required to validate this approach in children.