Which test differentiates neurogenic DI and primary polydipsia from nephrogenic DI?

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

Answer: (B)

Focused on how the body handles vasopressin. The key is measuring AVP and its stable surrogate copeptin, which reflects what the pituitary is releasing. In nephrogenic DI, the kidneys don’t respond to AVP, so the body pumps out more AVP/copeptin in an attempt to concentrate urine, leading to high AVP and copeptin levels. In neurogenic (central) DI, AVP production is deficient, so AVP and copeptin are low. In primary polydipsia, excess water intake suppresses AVP/copeptin release, so those levels are also low. Therefore, testing AVP together with copeptin can distinguish nephrogenic DI from the other two conditions and can help tell central DI from primary polydipsia.

Desmopressin tests and water deprivation have value in DI assessment but don’t separate all three scenarios as directly or reliably as measuring AVP and copeptin does.