The above figure summarizes the best data on use of serum copeptin level for diagnosis of diabetes inspidus and primary polydipsia. Copeptin derives from the precursor protein of AVP and has been correlated well with AVP production
What about in cases of increased ADH- such as SIADH? Data on that is poor and only one large study of >200+ patients showed that in hypovolemic hyponatremia ( appropriate ADH), the copeptin levels were >84pmol/L ( spec 90%, sens 23%) and SIADH fell in the 10-30pmol/L categories( not helping much) and hypervolemic and diuretic induced fell in 30-65pmol/L category. Use of this test is currently not recommended in diagnosis of increased AVP production disorders.
Finally in patients with nephrogenic syndrome of inappropriate anti diuresis, copeptin levels were are low or suppressed.
This article is an excellent summary of the use of copeptin in Na related disorders
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6850413/
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