Can the clots be treated?
Generally, the quicker that cases are identified, the better for the patient.
Treatment can involve a number of medicines and includes immunoglobulins - immune proteins - given through a drip.
Some patients will be sent home with new generation blood thinners and have regular monitoring.
According to blood specialist, Prof Adrian Newland, current evidence suggests 75% of people survive.
An expert panel of blood specialists in the UK have put together guidance for doctors, which is being constantly updated.