APS is a disorder which is often associated with certain autoimmune disorders, such as lupus, and with clinical manifestations of thrombosis, thrombocytopenia, or spontaneous fetal abortion. Elevated levels of anti-phospholipid antibodies are recognized clinically as serological markers for APS. Antibodies produced in APS are directed toward negatively charged phospholipids. APS patients have been found to produce anti-phospholipid antibodies in any combination of three immunoglobulin isotypes: IgG, IgM, and/or IgA. In general, IgG antibodies appear to be the most clinically relevant isotype. Both IgG and IgA anti-phospholipid antibodies appear to have stronger levels of correlation with thrombosis than IgM antibodies. Cardiolipin (CL) and phosphatidylserine (PS) appear to be the most common phospholipids to which antibodies react.
Laboratory criteria for APS include the presence of aCL antibody of IgG and/or IgM isotype in medium or high titer on 2 or more occasions at least 6 weeks apart. Also, the presence of lupus anticoagulant (LA) in plasma on 2 or more occasions at least 6 weeks apart, and / or antibodies to B2GPI cofactor or other phospholipids are lab criteria.