The hallmark of DI is a dilute urine in the face of Hypertonic Plasma with Hypernatraemia.
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In central DI, the urine osmolarity is often below 200 mosm/L, whereas in nephrogenic DI, the urine osmolarity is usually between 200 and 500 mosm/L.
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The diagnosis of DI is confirmed by noting the urinary response to fluid restriction. Failure of the urine osmolarity to increase more than 30 mosm/L in the first few hours of complete fluid restriction is diagnostic of DI.
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Once the diagnosis of DI is confirmed, the response to DDAVP (20mcg intranasally or 2 mcg IM, SC or IV) will differentiate central from nephrogenic DI.
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In central DI, the urine osmolarity increases by at least 50% almost immediately after DDAVP administration, whereas in nephrogenic DI, the urine osmolarity is unchanged.
