To determine the efficacy of a daily dose of 81 mg aspirin in primary thrombosis prevention in asymptomatic, persistently antiphospholipid antibody (aPL)-positive individuals (those with positive aPL but no vascular and/or pregnancy events).

The Antiphospholipid Antibody Acetylsalicylic Acid (APLASA) study was a multicenter, randomized, double-blind, placebo-controlled clinical trial in which asymptomatic, persistently aPL-positive individuals were randomized to receive a daily dose of 81 mg of aspirin or placebo. In a separate observational and parallel study, asymptomatic, persistently aPL-positive individuals who were taking aspirin or declined randomization were followed up prospectively.

In the APLASA study, 98 individuals were randomized to receive aspirin or placebo (mean +/- SD followup period 2.30 +/- 0.95 years), of whom 48 received aspirin and 50 received placebo. In the observational study, 74 nonrandomized individuals were followed up prospectively (mean +/- SD followup period 2.46 +/- 0.76 years); 61 received aspirin and 13 did not. In the APLASA study, the acute thrombosis incidence rates were 2.75 per 100 patient-years for aspirin-treated subjects and 0 per 100 patient-years for the placebo-treated subjects (hazard ratio 1.04, 95% confidence interval 0.69-1.56) (P = 0.83). Similarly, in the observational study, the acute thrombosis incidence rates were 2.70 per 100 patient-years for aspirin-treated subjects and 0 per 100 patient-years for those not treated with aspirin. All but 1 patient with thrombosis in either study had concomitant thrombosis risk factors and/or systemic autoimmune disease at the time of thrombosis.

Our results suggest that asymptomatic, persistently aPL-positive individuals do not benefit from low-dose aspirin for primary thrombosis prophylaxis, have a low overall annual incidence rate of acute thrombosis, and develop vascular events when additional thrombosis risk factors are present.