APS is often not recognised until someone has health problems caused by blood clots, such as repeated miscarriages, sudden lung blockages (pulmonary embolism), or a heart attack. To diagnose APS, doctors use three specific blood tests. At least one test must be positive on two occasions, at least three months apart, to confirm the diagnosis. Some people have these antibodies but never develop blood clots. Common risk factors that increase the chance of clotting include high blood pressure, obesity, smoking, hardening of the arteries (atherosclerosis), using birth control pills, or having another autoimmune disease like SLE.