Background: Diagnosis of antiphospholipid syndrome (APS) requires persistent presence of lupus anticoagulant (LAC), anticardiolipin (aCL) IgG/IgM, or anti-β2 glycoprotein I (aβ2GPI) IgG/IgM antibodies. Other antiphospholipid antibodies (aPL) such as antiphosphatidylserine/prothrombin antibodies (aPS/PT) are promising in assessment of thrombotic APS (TAPS).

Diagnosis of antiphospholipid syndrome (APS) requires the presence of a clinical criterion (thrombosis and/or pregnancy morbidity), combined with persistently circulating antiphospholipid antibodies (aPL). Currently, laboratory criteria aPL consist of lupus anticoagulant (LAC), anticardiolipin antibodies (aCL) IgG/IgM, and anti-β2 glycoprotein I antibodies (aβ2GPI) IgG/IgM. Diagnosis and risk stratification of APS are complex and efforts to standardize and optimize laboratory tests have been ongoing since the initial description of the syndrome. LAC detection is based on functional coagulation assays, while aCL and aβ2GPI are measured with immunological solid-phase assays. LAC assays are especially prone to interference by anticoagulation therapy, but strategies to circumvent this interference are promising. Alternative techniques such as thrombin generation for LAC detection and to estimate LAC pathogenicity have been suggested, but are not applicable yet in routine setting. For aCL and aβ2GPI, a lot of different assays and detection techniques such as enzyme-linked immunosorbent and chemiluminescent assays are available. Furthermore, a lack of universal calibrators or standards results in high variability between the different solid-phase assays. Other non-criteria aPL such as anti-domain I β2 glycoprotein I and antiphosphatidylserine/prothrombin antibodies have been suggested for risk stratification purposes in APS, while their added value to diagnostic criteria seems limited. In this review, we will describe laboratory assays for diagnostic and risk evaluation in APS, integrating applicable guidelines and classification criteria. Current insights and hindrances are addressed with respect to both laboratory and clinical implications.

Background: Antiβ2glycoprotein I (aβ2GPI) and anticardiolipin (aCL) IgG/IgM show differences in positive/negative agreement and titers between solid phase platforms. Method-specific semiquantitative categorization of titers could improve and harmonize the interpretation across platforms. Aim: To evaluate the traditional 40/80-unit thresholds used for aCL and aβ2GPI for categorization into moderate/high positivity with different analytical systems, and to compare with alternative thresholds. Material and methods: aCL and aβ2GPI thresholds were calculated for two automated systems (chemiluminescent immunoassay [CLIA] and multiplex flow immunoassay [MFI]) by receiver operating characteristic curve analysis on 1108 patient samples, including patients with and without antiphospholipid syndrome (APS), and confirmed on a second population (n = 279). Alternatively, regression analysis on diluted standard material was applied to identify thresholds. Thresholds were compared to 40/80 threshold measured by an enzyme-linked immunosorbent assay (ELISA). Additionally, likelihood ratios (LR) were calculated. Results: Threshold levels of 40/80 units show poor agreement between ELISA and automated platforms for classification into low/moderate/high positivity, especially for aCL/aβ2GPI IgG. Agreement for semiquantitative interpretation of antiphospholipid antibodies (aPL) IgG between ELISA and CLIA/MFI improves with alternative thresholds. LR for aPL IgG increase for thrombotic and obstetric APS based on 40/80 thresholds for ELISA and adapted thresholds for the other systems, but not for IgM.