The antiphospholipid syndrome (APS) is sometimes called 'sticky blood syndrome'. This is because people with it have an increased tendency to form clots in blood vessels (also known as thromboses). Any blood vessel can be affected including the veins, which are thin walled and take blood towards the heart, and the arteries, which have thick muscular walls, and take blood away from the heart. As a result of this clotting tendency these patients may develop repeated clots, eg in the veins of the leg (deep vein thrombosis), or in the arteries supplying the brain, causing a stroke. In pregnancy, the placenta can be affected by small clots, and there is an increased risk of miscarriage, particularly in mid-pregnancy.

The symptoms relate to abnormal clotting. Most commonly clots can develop in the veins of the legs (deep vein thrombosis), resulting in a swollen, painful leg, with the swelling usually starting in the calf. These clots in the leg may happen once, or several times. Clots may also occur in the lung (pulmonary embolus), sometimes following a clot in the leg, which breaks off and travels in the circulation until it lodges in part of the blood supply to the lung. A clot in the lung may lead to shortness of breath, chest pain made worse by breathing in, or coughing up a small amount of blood. Some clots occur in patients soon after starting on the oral contraceptive pill (oestrogen-containing types).

Veins in other parts of the body may clot, including the eye, the kidney, the liver, and the adrenal gland.

Depending on where it occurs, clotting within an artery can result in a stroke, heart attack, or blockage of the blood supply to a limb, possibly leading to gangrene. Such arterial diseases occur commonly from atherosclerosis in old age, but in the antiphospholipid syndrome they can happen at a much younger age. Hence a patient under 50 who has a stroke or a heart attack should be tested for the antiphospholipid syndrome.

Women with antiphospholipid syndrome are at increased risk of miscarriage. Clots may form in the placenta leading to an inadequate blood supply to the foetus. Miscarriages in antiphospholipid syndrome usually occur in the second or third trimester (week 12 or later), but can sometimes occur in the first trimester. Fifteen per cent of women with three or more consecutive miscarriages have positive antiphospholipid syndrome tests. There are patients with antiphospholipid syndrome who have successful pregnancies without treatment, though there is a higher risk of complications towards the end of pregnancy, including pre-eclampsia, intrauterine growth retardation (slowing down of the baby's growth in the uterus), and premature delivery.

Patients with antiphospholipid syndrome report a higher incidence of headaches, including migraines. A proportion of patients have memory loss, or more subtle abnormalities. Epilepsy is commoner in antiphospholipid syndrome. There is a higher incidence of chorea, also known as St.Vitus' Dance (abnormal sudden jerky movements).

Patients with secondary antiphospholipid syndrome may report symptoms of the underlying disease. For example, patients with underlying systemic lupus erythematosus may report features including a rash on the cheeks, skin reaction to sunlight, joint pains, mouth ulcers, excessive hair loss, dry eyes, cold and bluish fingers, and sometimes sharp chest pains.