Some people from other team saying the patient has SIADH...

Maybe.

The aim of DDAVP-HTS treatment protocol is to gain control of patient's SNa quickly and raise it in a controlled manner. This approach doesn't in itself facilitate the diagnosis of the underlying cause of hypoNa

As patients commonly have more than one cause of hypoNa, and their kidneys will respond to any treatment given by the clinicians or other condition change (e.g. volume or haemodynamic status), it isn't uncommon to observe that the urine output and contents (UOsm and electrolytes) change quite significantly during the course of treatment even with the traditional approach.

Similarly, DDAVP-HTS leads to some (predictable) change in patient's urine chemistry as it creates an iatrogenic SIADH state (they are given DDAVP!!) so it is expected that the urine chemistry profile appears exactly like SIADH after the initiation of DDAVP-HTS protocol.

Therefore, it is important to workup for the cause before the biochemical parameters are affected by the protocol, thus the importance of the initial blood & urine samples before protocol initiation.