The antiphospholipid syndrome (APS) is a systemic autoimmune 
disorder characterized by the main clinical features of venous and/or 
arterial thrombosis (intravascular blood clotting), thrombocytopenia, 
recurrent fetal loss, and moderate-to-high titers of antiphospholipid 
(aPL) antibodies, lupus anticoagulant, anti-cardiolipin antibodies, 
and anti-beta-2-glycoprotein I antibodies (refs 1-2).  aPL antibodies 
are autoantibodies that react with negatively-charged phospholipids 
such as cardiolipin.  They are often found in the sera of patients 
with systemic lupus erythematosus and related diseases and are typical 
for the development of secondary APS.  The presence of 
antiphospholipid antibodies in patients with no other autoimmune 
disease is characteristic of primary APS.  Anti-cardiolipin antibodies 
are found in 80-90% of patients with APS.  However, the presence of 
anti-cardiolipin antibodies are not specific to APS, because these 
antibodies may be detected in other autoimmune diseases, following 
administration of certain drugs, and in the aftermath of infectious 
disease (e.g., hepatitis C, syphilis, and infectious mononucleosis).  
Beta-2-glycoprotein I, also known as apolipoprotein H, inhibits the 
intrinsic coagulation pathway and is involved in the regulation of 
blood coagulation.  Beta-2-glycoprotein I has been shown to be the 
primary autoantigen for anti-cardiolipin antibodies.  Antibodies to 
beta-2-glycoprotein I are more specific to APS than anti-cardiolipin 
antibodies.  Due to the heterogenous nature of aPL antibodies, 
combined measurements of anti-cardiolipin and anti-beta-2-glycoprotein 
I antibodies may provide better sensitivity than either assay alone.

The criteria for defining APS as described by the Miyakis/Sapporo 
criteria (International Consensus Statement on an Update of the 
Classification Criteria for Definite Antiphospholipid Syndrome, 
Journal of Thrombosis and Hemostasis 2006; 4:295-306) are as follows:

Clinical criteria:
1. Vascular thrombosis
   One or more clinical episodes of arterial, venous or small vessel
   thrombosis
2. Pregnancy morbidity
   (a) One or more unexplained deaths of a morphologically normal fetus
       at or beyond the 10th week of gestation
   (b) One or more pre-term births of a morphologically normal neonate
       before the 34th week of gestation because of: (i) eclampsia or
       severe pre-eclampsia or (ii) recognized features of placental
       insufficiency
   (c) Three or more unexplained consecutive spontaneous miscarriages
       before the 10th week of gestation, with maternal anatomic or
       hormonal abnormalities and paternal and maternal chromosomal
       causes excluded

Laboratory criteria:
1. Lupus anticoagulant present in plasma, on two or more occasions at
   least 12 weeks apart
2. Anticardiolipin antibody of immunoglobulin IgG and/or IgM isotype in
   serum or plasma, present in medium or high titer (i.e. >40GPL units
   or MPL units, or > the 99th percentile), on two or more occasions,
   at least 12 weeks apart
3. Anti-beta-2-glycoprotein I antibody of IgG and/or IgM isotype in
   serum or plasma (in titer >the 99th percentile), present on two or
   more occasions at least 12 weeks apart

APS is present if at least one of the clinical criteria and one of the 
laboratory criteria above are met.

The presence of IgG antibodies to cardiolipin or beta-2-glycoprotein I 
at levels greater than 99th percentile is part of the SLICC (Systemic 
Lupus International Collaborating Clinics) Classification Criteria for 
Systemic Lupus Erythematosus immunologic criteria (ref. 3).

References:
1. International Consensus Statement on an Update of the Classification
   Criteria for Definite Antiphospholipid Syndrome, Journal of
   Thrombosis and Hemostasis 2006; 4:295-306.

2. Devreese K and Hoylaerts M.  Clinical Chemistry 2010; 56:930-940.

3. Petri M et al.  Arthritis and Rheumatism 2012; 64(8):2677-2686.