During the blood test, the blood is screened for clotting, bleeding/blood count, blood chemistry, liver function, and HIV. Generally the test is to determine your general state of health and whether you are fit and well enough to go through with the surgery.
A responsible clinic should ask you for your medical history during the early consultation stage.
If you have any doubts about your general health then it might be wise to have a blood test with your GP prior to any surgery just for your own piece of mind to ensure that you have no abnormal levels that might postpone the surgery......just a thought.