Prevalence and significance of anti-prothrombin (aPT) antibodies in patients with Lupus Anticoagulant (LA)

Objective: Anti-prothrombin (aPT) antibodies have been found in Lupus Anticoagulant (LA) positive patients. Their prevalence and relative contribution to thromboembolic risk in LA-positive patients is not well defined. The aim of this study was to determine their presence and association with thromboembolic events in a large series of patients with confirmed LA. Methods: Plasma from LA-positive patients was collected at Thrombosis Centers and sent to a reference central laboratory for confirmation. Positive plasma was tested using home-made ELISA for the presence of aPT and anti-β 2GPI antibodies. Results: LA was confirmed in 231 patients. Sixty-one of 231 (26%, 95%CI 22-33) LA positive subjects were positive for IgG aPT and 62 (27%, 95% CI 21-33) were positive for IgM aPT antibodies. Clinical features of Antiphospholipid Syndrome (APS) were not associated with the presence of IgG aPT [43 APS in 61 (70%) positive and 109 APS in 170 (64%) negative IgG aPT subjects, p = ns] or IgM aPT. Rate of positivity of IgG and IgM aβ 2GPI was significantly higher than that of IgG and IgM aPT. Clinical events accounting for APS occurred in 97 of 130 (75%) IgG aβ 2GPI positive and in 55 of 101 (54%) IgG aβ 2GPI negative patients (OR 2.4, 95% CI 1.4 to 4.3, p = 0.002). No significant association with clinical events in patients positive for both IgG aPT and IgG aβ 2GPI as compared to those positive for one or another test was found. When patients negative for both IgG aPT and IgG aβ 2GPI (LA positive only) were compared with remaining patients, a significantly lower association with clinical events was found (OR = 0.4, 95% CI: 0.2 to 0.7, p = 0.004). Conclusions: As compared to IgG aβ 2GPI, the prevalence of IgG aPT in patients with LA is significantly lower and not associated with the clinical features of APS.