Summary

Recent reports have described the factor XIII A subunit (FXIII-A) Val34Leu polymorphism as a protective factor against venous and arterial thrombosis. The aim of this study was to investigate the association between the FXIII-A Val34Leu polymorphism, its interaction with fibrinogen concentration, and thrombosis in patients with antiphospholipid antibodies (aPL). We included 172 consecutive patients with aPL: 88 with primary antiphospholipid syndrome (APS), 38 with APS associated with systemic lupus erythematosus (APS-SLE), 32 with SLE and aPL but without APS (SLE-aPL), and 14 asymptomatic individuals with aPL (A-aPL). The FXIII-A Val34Leu polymorphism was assessed by polymerase chain reaction techniques. We found no significant differences in FXIII-A Leu34 allele frequencies between primary APS (allele frequency 0.22), APS-SLE (0.23), SLE aPL (0.22) and A-aPL (0.32) patients, or between patients with (0.21) and without thrombosis (0.26). FXIII-A Leu34 allele frequencies were significantly lower in patients with thrombosis and those in the upper fibrinogen tertile (>3.40 g/l) (allele frequency 0.07) compared with patients without thrombosis in the upper fibrinogen tertile (0.29) and patients with (0.29) and without (0.25) thrombosis in the mid- and lower fibrinogen tertiles. The FXIII-A Leu34 allele had a protective effect against thrombosis in patients in the upper fibrinogen tertile (odds ratio [OR]=0.20, 95% confidence interval [CI] 0.07–0.60) but not in those in the other tertiles (OR=1.20, 95% CI 0.67–2.16). The FXIII-A Leu34 allele seems to have a protective effect on the development of thrombosis in patients with aPL, but only in those with high plasma fibrinogen values.