Veins and arteries vary in size so it may be harder to take a blood sample from one person than another. Other slight risks from having blood drawn are may include:
Fainting or feeling light-headed
Hematoma (blood accumulating under the skin)
Infection (a slight risk any time the skin is broken)
This test is most often done on people who have bleeding problems. The risk of excessive bleeding is slightly greater than for people without bleeding problems.
When you bleed, the body starts a series of activities that help the blood clot. This is called the coagulation cascade. The process involves special proteins called coagulation factors (factor VII is a coagulation factor).
Each factor's reaction triggers the next reaction. The final product of the coagulation cascade is the blood clot. Blood clots may not form normally if any one of the clotting factors is abnormally low.