Prevalence of anti–A2 IgG and IgM in patients with antiphospholipid syndrome. (A) A2 Index. Sera from healthy controls (Healthy), patients with lupus without thrombosis (SLE), patients with thrombosis related to nonimmune risk factors (Nonimmune Thrombosis), and patients with antiphospholipid syndrome (APS) were analyzed for anti-A2 antibodies by ELISA as described in “Patients, materials, and methods.” For each patient, the A₄₀₅ value for either IgG or IgM was expressed as a ratio to the mean A₄₀₅ value for 30 simultaneously evaluated healthy control samples. The sum of the IgG and IgM ratios (A2 index) for each individual patient is represented as a single point. Shown in red are the A2 index values for patients with either IgG or IgM titers that exceed the mean control value by more than 3 SDs. Bars indicate mean values for each group. (B) Prevalence of anti-A2 antibodies (3 SDs) in patient subgroups. For each group, the percentage of patients with an anti–A2 IgG or IgM titer that exceeds the mean control value by more than 3 SDs is shown. Absolute optical density values are 1.16 for IgG and 0.15 for IgM. P values for anti-A2 antibodies in antiphospholipid syndrome (APS), compared with healthy controls (Healthy), patients with systemic lupus erythematosus without thrombosis (SLE), or individuals with nonimmune thrombosis (Thrombosis) were highly significant (P < .001, P < .001, and P < .017, respectively). Antibody prevalence is also shown for APS subgroups as defined by Sapporo Criteria.¹