Acute Blood Loss

Acute loss of blood does not lead to any immediate change in the full blood count: haemodilution takes some time to occur, hence the full blood count can never be used to decide whether or not someone has suffered a significant acute haemorrhage. Physical examination will tell you this, the two most reliable signs of intravascular volume depletion being postural hypotension (lying and sitting) and a reduced jugular venous pressure.

Fig 1: Postural Hypotension

Fig 2: Jugular venous pressure (JVP)

Source: Fig 1, Fig 2,