**2. Epidemiology**

APA can be detected in the absence of thrombosis or pregnancy morbidity or other systemic autoimmune diseases. During ongoing infectious disease, during treatments with a variety of drugs and even in healthy individuals, APA positivity may occur. The prevalence of APA ranges from 1% to 10% in the general population, 16% in patients with rheumatoid arthritis, and 30% to 40% in patients with SLE(Petri M 2000, Lim W et al 2006). The prevalence of positive tests for lupus anticoagulant and anticardiolipin antibody in a normal population has been reported in several studies. Because of the non-Gaussian distribution of anticardiolipin antibody levels in normal subjects, the cut-off points between normal and abnormal results is difficult to determine. One study reported IgG and IgM anticardiolipin antibodies in approximately 5% of normal individuals, although only 2% had persistently elevated levels on repeat testing. Shi and colleagues detected anticardiolipin antibodies in 6% of normal blood donors, respectively, and detected lupus anticoagulant activity by kaolin clotting time in 4%Shi W 1993). The prevalence of anticardiolipin antibody appears to increase with age.

The prevalences of elevated levels of IgG and IgM anticardiolipin antibody in healthy pregnant women were 2% to 3% and 4%, respectively(Harris EN 1991, Aoki K 1994, Lockshin MD 1997). Most of these were low titer; only 0.2% were high titer. In other studies, the incidence of anticardiolipin antibodies in pregnant individuals ranged from 1% to 2% and lupus anticoagulant 1% to 4%(Petri M 2000).

When the patient does not exhibit any other symptom that would allow the diagnosis of another associated autoimmune disease, the antiphospholipid syndrome is considered primary, or isolated. The term 'secondary' APLS is sometimes used for patients suffering from another autoimmune or inflammatory disease.
