The first official classification criteria for anti-phospholipid syndrome (APS) were drafted in 1998 at a workshop at the 8th International Symposium on Anti-phospholipid Antibodies in Sapporo, Japan (Sapporo criteria; Wilson et al., Arthritis & Rheumatism 1999). According to these criteria, APS can be considered proven if at least one clinical and one serological criterion are met. Clinical criteria include vascular thrombosis, which must be established according to the stipulated criteria, and pregnancy complications such as premature births, spontaneous abortions and eclampsia.

When the criteria were updated in 2004 (Miyakis criteria; Miyakis et al., Journal of Thrombosis and Haemostasis 2005) antibodies against β2 glycoprotein 1 were added. Fulfilment of the criteria now encompasses at least one of the following three parameters: antibodies against cardiolipin (ACA; IgG or IgM) or β2 glycoprotein 1 (anti-β2GP1; IgG or IgM) or a positive lupus anticoagulant (LA) test. The latter is a coagulation test. According to official recommendations the serological criteria for APS diagnosis are only fulfilled when the result is confirmed 12 weeks later in a further test. A further update of the classification criteria in 2012 (Lakos et al., Arthritis & Rheumatism 2012) included the additional recommendation that when IgG and IgM tests for ACA or anti-β2GP1 IgG are negative, IgA should be tested as well.

In serological APS diagnostics autoantibodies of several immunoglobulin classes (IgAGM) can occur simultaneously, although often only one Ig class is detected. The association of particular immunoglobulin classes (IgAGM) with particular clinical parameters is controversially discussed.

Since around 10% of the healthy normal population exhibit anti-phospholipid antibodies (APLA) in the form of ACA or LA and these antibodies can also be induced by infections or specific medications (e.g. procainamide and hydralazine), it is important to connect the serological results with clinical criteria.