Current use of aspirin and antithrombotic agents in the United States among outpatients with atherothrombotic disease (from the REduction of Atherothrombosis for Continued Health [REACH] Registry). 
Despite its proven efficacy, low cost, and wide availability, aspirin remains underused.  We examined current aspirin use and determined factors that influence its use among outpatients in the United States (US).  The REduction of Atherothrombosis for Continued Health (REACH) Registry is an international, prospective, longitudinal study of >68,000 outpatients with established atherothrombosis or >or=3 atherothrombotic risk factors.  The rates of aspirin use were compared in various patient subgroups.  Multivariate logistic regression models were constructed to determine the factors influencing the baseline use of aspirin and other antithrombotic agents in the US population.  Approximately 70% of 25,686 US outpatients were treated with aspirin, with greater use in the Midwest and among men, whites, and those aged <65 years.  Among aspirin users, 18% took other antiplatelet agents and 6% took oral anticoagulants.  Low-dose aspirin (<or=100 mg/day) was used in approximately 2/3 of aspirin users.  Of patients not taking aspirin, 1/2 were receiving oral anticoagulants or other antiplatelet agents.  However, 15% of patients used no antithrombotic agent at all.  Female gender, current smoking, or having diabetes mellitus were predictors of a lack of antithrombotic use; white race, atrial fibrillation or vascular disease, the use of other risk-reducing medications, or treatment by a cardiologist were associated with a greater likelihood of receiving antithrombotic therapy.  In conclusion, approximately 1/4 of US patients with vascular disease are not treated with aspirin for secondary prevention, and 15% are not treated with any antithrombotic agent.