Introduction: Copeptin is the C-terminal fragment of proAVP and provides an alternative measure of AVP secretion. While direct measurement of AVP during hypertonic saline infusion has been recommended as a diagnostic test for diabetes insipidus (DI), the number of reports examining the utility of copeptin in this context is limited. Here we describe a series of cases where measurement of copeptin during saline infusion has contributed to the diagnosis of patients with polyuria/polydipsia syndrome.
Methods: Biochemical and clinical data were collated for 9 cases where copeptin was measured during hypertonic saline infusion for the investigation of polyuria/polydipsia. Data collected included random serum and urine osmolality, serum copeptin and osmolality during saline infusion, other endocrinology tests, pituitary imaging and clinical presentation/history. The final diagnosis made by a Consultant Endocrinologist in view of all test results and clinical presentation was also recorded.
Results: A diagnosis of primary polydipsia was made in 6 cases. In these cases the maximum serum copeptin concentration during saline infusion was between 9.0 and 28.0 pmol/L. A diagnosis of central DI was made in a further 2 cases. In these 2 cases the maximum serum copeptin during saline infusion was <2.3 pmol/L. In the final case the differential diagnosis after both water deprivation/ddAVP challenge and hypertonic saline infusion was between primary polydipsia and partial nephrogenic DI. In this case the maximum copeptin during saline infusion was much higher than in the cases of primary polydipsia at 125 pmol/L, suggesting a degree of AVP resistance.
Conclusions: The results of this small case series demonstrate the potential of serum copeptin under osmotic stimulation as a diagnostic test for DI. Additional data is being collated to further determine the diagnostic performance of the hypertonic saline infusion test with copeptin measurement. This test has potential advantages over the conventional water deprivation test.