Antiphospholipid antibodies include anticardiolipin antibodies (aCL), anti-beta2GP1 antibodies, and lupus anticoagulant. Approximately 10% of the normal population has low titers, usually positive for transient anticardiolipin antibodies, while the rate of positivity for moderate to high potency anticardiolipin antibodies or lupus anticoagulant is less than 1%. About 10-40% of patients with SLE and about 20% of patients with rheumatoid arthritis are positive for antiphospholipid antibodies. APL positivity is seen in 25% of patients with unexplained stroke aged less than 45 years. APL is positive in 20% of female patients with three or more pregnancy losses. APL was positive in 14% of patients with recurrent venous thrombotic events. Lupus anticoagulant was measured by a modified Russell’s viper venom dilution test. It includes the Lupo test II and the Lucor test, both of which have a normal ratio of 1.0 to 1.2, when an elevated ratio indicates the presence of lupus anticoagulant material. There is a stronger correlation between lupus anticoagulants and thrombosis and morbid pregnancy compared to anti-cardiolipin antibodies and anti-β2GP1 antibodies. Positive APL antibodies such as LA and ACL antibodies are of greater clinical significance when their titers are high.