Antiphospholipid syndrome (APS) is a thrombo-inflammatory disease driven by antiphospholipid antibodies (aPL). By recognizing phospholipid surfaces and phospholipid-binding proteins, aPL can induce thrombosis, pregnancy morbidity, and other autoimmune and inflammatory manifestations. Adequate diagnosis of APS is important as APS patients often need dedicated follow-up and anticoagulant treatment with vitamin K antagonists. The clinical features are not specific for APS, and thus APS diagnosis is dependent on accurate detection and interpretation of aPL. Numerous aPL are described, but only three of them are currently considered to be diagnostic, namely lupus anticoagulant (LA), anticardiolipin antibodies (aCL) IgG/IgM and anti-β2-glycoprotein I antibodies (aβ2GPI) IgG/IgM.

LA analysis is performed with two clotting assays in a three-step procedure, while aCL, aβ2GPI, and aPS/PT are measured with dedicated solid-phase immunological assays. Therefore, the laboratory diagnosis of APS is cumbersome as multiple laboratory tests are required for aPL assessment. The diagnostic workup is further complicated by potential interferences and lack of standardization/harmonization between assays. 

Progress has been made on the standardization of measurement and interpretation of LA and immunological assays for aCL and aβ2GPI. However, LA measurement remains a complicated procedure with many pitfalls and interfered by anticoagulant therapy. Solid phase assays for aCL and aβ2GPI still show inter-assay differences. These methodological issues make the laboratory diagnosis of APS challenging. The thrombin generation assay (TGA) might offer an alternative to traditional aPL testing. The TGA measures both thrombin formation and inhibition, which represent a significant part of the overall coagulation process. Therefore, TGA reflects more closely what occurs in vivo compared to clotting-based assays by measuring the dynamic processes of thrombin generation.

In the interpretation of aPL results, antibody profiles help in identifying patients at risk. Non-criteria aPL, such as antibodies against the domain I of beta2-glycoprotein (aDI) and antiphosphatidylserine-prothrombin (aPS/PT) antibodies have been studies in the last years and may be useful in risk stratification of APS patients.  But, aDI and aPS/PT are not included in the current diagnostic criteria and testing in daily practice is not recommended as these antibodies have no added value in the diagnosis of APS.

There is still need for optimalisation of aPL testing. These include studies on aPS/PT and aDI antibodies, comparability of different solid phase assays (ELISA versus non-ELISA assays),  semiquantitative reporting of titers, and the role of IgM and IgA isotypes. In addition, the clinical role of moderate and high titer thresholds and the cutoffs calculated by the 99^th percentile need further study. More optimalization initiatives will benefit the diagnosis of APS and contribute to proper and accurate interpretation of the results.