Diseases of the microcirculation

Antiphospholipid Syndrome (APS) is a systemic autoimmune disorder associated with abnormal blood clotting. In patients with APS, the immune system creates autoantibodies known as antiphospholipid antibodies (aPL) that increase risk of blood clots by targeting the blood vessels and clotting proteins. 

Antiphospholipid antibodies occur in otherwise healthy individuals or in patients with autoimmune disorders such as systemic lupus erythematosus (SLE) or rheumatoid arthritis (RA). Individuals with positive aPL may or may not develop clinical problems, so positive aPL test alone without any clinical problems is not enough to confirm the APS diagnosis. 

Based on the International APS Classification Criteria a clinical event (blood clot or pregnancy complications) and one of the following blood tests are required for the APS diagnosis (patients should undergo at least two blood tests, which should be spaced three months apart to show persistent levels of aPL). Commonly used tests include:

Lupus anticoagulant (LA) test
Anticardiolipin antibody (aCL) test
Anti-Beta-2 glycoprotein-I (aB2GPI) test.

Positive LA as well as higher levels (> 40 units) of aCL and aB2GPI correspond to an increased risk of an aPL-related events whereas lower levels (especially <20 units) are clinically less significant. Patients who are positive for all three of the aPL blood tests seem to have a higher likelihood of an aPL-related event than those with only one or two positive tests.

Major clinical problems associated with APS include blood clots in the veins or arteries, and pregnancy losses. Deep vein thrombosis is the most common type of blood clot to form in the veins, and a stroke is the most common type of blood clot to occur in the arteries. Some of the other clinical problems related to aPL include livedo reticularis (lacey purple pattern on skin), heart valve disease, low platelet count, anemia, and kidney disease.