I take DVT very seriously after one of our senior managers nearly died on a flight from Japan to Hawaii. If his wife hadn't nagged him into asking the FA if he was in trouble, he might not have survived.
The FA's on the flight took action (they are a lot of DVT problems on those flights) and alerted the pilots. An ambulance met the plane.
It was no easy fix. He spent 5 weeks in ICU and in hospital in Hawaii and then another month in an apartment there before the docs would let him fly home.
He told us that he felt 'bad' and had pain but thought it was an existing back problem. His wife would not let him go back to sleep on the plane and saved his life!
He wanted to sleep it off!