*2.2.1 Anti-prothrombin antibodies*

APT antibodies are detected by ELISA using a purified prothrombin as antigen coated onto irradiated plates [28] or phosphatidylserine/prothrombin complex [29]. Although a correlation between the two assays have been reported, these antibodies differed either in affinity or in epitopes that they recognized [30]. The ones directed against anti-phosphatidylserine/prothrombin complex (aPS/PT) seems having a closer association with APS and LA activity than with antibodies to prothrombin alone [31, 32]. aPS/PT have been reported to be significantly associated with both thrombotic and obstetric manifestations of APS [33–35]. Moreover, since they have

been shown to be closely related to LA, have been proposed as a surrogate test for and as an additional serologic marker of APS, to be performed with other aPL tests to improve diagnosis [36, 37].

### *2.2.2 Anti-domain I antibodies*

A subgroup of anti-β2GPI, those directed to domain I of the molecule [15], have been reported to be strongly associated with thrombosis and LA in APS patients while those directed to domain IV/V are less frequent [25, 26]. Recently has been suggested that the ratio between anti-β2GPI-DI and anti-β2GPI-DIV/V IgG can provide a better profile of anti-β2GPI antibodies linked to APS and antibodies occurring in other pathologic condition [38].

To improve risk prediction of recurrent thrombosis and pregnancy loss the Global Anti-Phospholipid Syndrome Score (GAPSS) was developed, considering the aPL profile, conventional cardiovascular risk factors, and autoimmune antibody profile. Validated in APS and in SLE patients, a high GAPSS score predicted thrombosis better than aPLs alone [39, 40]. Recently, the GAPSS score has been shown to be a useful tool for predicting a higher likelihood of favorable pregnancy outcome in pregnant women treated with conventional therapy [41].
