Relationship between antiphosphatidylserine/prothrombin and conventional antiphospholipid antibodies in primary antiphospholipid syndrome.

BACKGROUND: Antiphosphatidylserine/prothrombin complex (aPS/PT) antibodies are emerging as an important marker for antiphospholipid syndrome (APS). We aimed to compare their performance with that of conventional antiphospholipid antibodies (aPL) such as lupus anticoagulant (LA), anticardiolipin (aCL), and anti-β2-glycoprotein I (anti-β2GPI) in APS and to assess their frequency in APS-negative (APS-ne) patients.
METHODS: We considered 160 APS patients and 128 APS-ne patients with clinical criteria for APS but tested negative for conventional aPL. Immunoglobulin (Ig)G/IgM aPS/PT, IgG/IgM aCL, and IgG/IgM anti-β2GPI were detected using ELISA assay and LA with a series of coagulation tests.
RESULTS: IgG aPS/PT were significantly associated with IgG aCL, IgG anti-β2GPI, and LA (p<0.0001 for all). IgM aPS/PT were significantly associated only with LA (p<0.0001) instead. There was a significant correlation between IgG aPS/PT and both IgG aCL and IgG anti-β2GPI levels (ρ=0.42 and ρ=0.40, respectively). Both IgG aPS/PT and IgM aPS/PT positivity significantly correlated with LA (ρ=0.44 and ρ=0.5, respectively). IgG and IgM aPS/PT were significantly more frequent in triple than in double and in single positivity (p<0.0001). According to multivariate analysis, IgG and/or IgM aPS/PT were independent risk factors for LA. APS/PT antibodies were found in 9.4% of the APS-ne patients vs. 2% of healthy control (p=0.043); those antibodies were significantly more frequent in the thrombosis with respect to the pregnancy morbidity subset (p=0.01).
CONCLUSIONS: Our data attribute a clinical relevance to both IgG and IgM aPS/PT antibodies. In particular, the significant prevalence of aPS/PT in APS-ne patients suggests including them as additional laboratory criterion for APS.