Doc confirmed localized petechia rash on shoulder, said no worry as there was a history of minor trauma as i had hit it. said blood tests not needed?

Trauma no blood test: If there is history of trauma it can cause petechae and you do not need blood test the most common cause of petchae is physical trauma and it will clear up by itself.In a case like yours it is up to the examining doctor to decide if you need blood test for platlet count.Some doctors will order it just to be on the safe side.Some will only do it if there is recurrence of petechae.