Introduction: Copeptin is the C-terminal fragment of proAVP and secreted in equimolar amounts with AVP. While AVP is unstable in vitro and has proved difficult to measure in clinical practice, copeptin is relatively stable and can be measured using an automated immunoassay. Therefore copeptin measurement offers potential as a more practical alternative to the direct measurement of AVP in the investigation of polyuria/polydipsia.
Methods: AVP, copeptin, and plasma osmolality were measured in parallel using plasma samples from 15 patients undergoing a hypertonic saline stress test. AVP was measured using in-house radioimmunoassay and copeptin using the Brahms Kryptor immunoassay. AVP and copeptin values were correlated with plasma osmolality using an in-house normogram allowing comparison of AVP response to a reference population. AVP and copeptin responses to hypertonic stress were compared.
Results: In 12 cases the AVP response was considered normal. In three cases the AVP response was considered sub-normal, with AVP failing to demonstrate sufficient response to an osmotic stimulus, consistent with cranial diabetes insipidus. In all 15 cases the response of copeptin was equivalent to that of AVP. In all 12 cases with a normal AVP response, the peak copeptin was at least 12.7 pmol/l (range 12.773.4) and copeptin concentration was at least 4.6 pmol/l in samples with osmolality ≥300 mOsm/l. In the three cases with a sub-normal AVP response consistent with diabetes insipidus, two were associated with very poor copeptin response (maximum copeptin ≤2.6 pmol/l). In the remaining case both AVP and copeptin responses were below the expected normal range.
Conclusions: The results of this small study suggest that copeptin provides equivalent information to AVP when measured during hypertonic saline infusion tests. Further study is required to determine the diagnostic performance of copeptin measurement during hypertonic saline infusion in the investigation of polyuria/polydipsia.