Which test differentiates neurogenic DI from primary polydipsia?

• A. Desmopressin Test
• B. AVP and Copeptin
• C. 3% hypertonic saline infusion and Copeptin
• D. Water deprivation test

Answer: (C)

The key idea is how the body’s vasopressin (AVP) system responds to an osmotic challenge. Neurogenic (central) diabetes insipidus is a failure to release AVP, so even when osmolality rises, AVP—and its surrogate copeptin—stays low. Primary polydipsia, on the other hand, involves excessive water intake that suppresses AVP at baseline, but the AVP axis remains capable of responding to increased osmolality; copeptin should rise when osmolality is raised.

Giving a controlled 3% saline infusion raises serum osmolality and provokes AVP release. Measuring copeptin during this osmostress test shows whether the pituitary can respond. A normal or expected rise in copeptin suggests a functioning AVP system consistent with primary polydipsia, whereas a blunted or absent rise points to neurogenic DI.

This approach is more informative for distinguishing neurogenic DI from primary polydipsia than tests like the desmopressin test (which differentiates central vs nephrogenic DI) or a standard water deprivation test (which can be less specific and slower).