Endocrine Abstracts

JOINT2456

Introduction: Central diabetes insipidus is a condition caused by vasopressin deficiency, which results in an inability to concentrate urine and consequently manifests with polyuria and polydipsia. In most cases it arises after trauma, tumours or autoimmune processes¹. The water deprivation test is the most widely used test. The test is considered positive if peak serum osmolality is > 295 mmol/kg or if peak urine osmolality is < 300 mmol/kg or if the copeptin dosage is < 2.6 pmol/L².

Case Study: 38-year-old patient with polyuria and polydipsia that had arisen about two years previously and worsened over the last year. The patient reported a fluid intake of about 8 litres per day. No previous history of trauma. Blood tests showed normal electrolyte values, but urine tests showed a lower specific gravity than normal. An MRI with contrast of the pituitary gland showed a normal gland. Therefore, considering the patient’s symptoms, we decided to perform a water deprivation test, lasting approximately 14 hours. Blood samples were taken every two hours for dosing copeptin, serum osmolarity, sodium, as well as urine samples for urinary osmolarity and urine specific gravity. From the start to the end of the test: the patient’s body weight decreased by approximately 1.2 kg (92.8 Kg vs 91.6 Kg). Serum osmolarity increased from 288 mOsm/Kg to 301 mOsm/kg and urinary osmolarity increased from 487 mOsm/Kg to 611 mOsm/Kg. The urine specific gravity increased from 1005 to 1014 g/L; sodium values increased slightly during the test from 141 to 144 mmol/L; and, finally, the copeptin value remained below 2 pmol/lthroughout the test. Treatment The patient started therapy with desmopressin acetate 90 mg/day. Thake Home Message Vasopressin is an unstable hormone and it’s difficult to measure it in laboratory in contrast to copeptin, the c-terminal fragment of the vasopressin precursor, which provides a reliable indirect measure of bioactive hormone³. Although in our clinical case already the serum osmolarity value allowed us to make a diagnosis, the copeptin assay was essential to make a definite diagnosis of central diabetes insipidus.

Bibliography: 1. Tomkins et al. Diagnosis and Management of Central Diabetes Insipidus in Adults. J Clin Endocrinol Metab. 2022.

2. Rowe et al. Use of copeptin in interpretation of the water deprivation test. Endocrinol Diabetes Metab. 2023.

3. Morgenthaler et al. Assay for the measurement of copeptin, a stable peptide derived from the precursor of vasopressin. Clin Chem. 2006.