The short life span of mature blood cells requires their continuous replacement, a process termed hematopoiesis. New cell production must be responsive to both basal needs and situations of increased demand. For example, red blood cell production can vary over more than a fivefold range in response to anemia or hypoxia. White blood cell production increases dramatically in response to a systemic infection, and platelet production can increase severalfold when platelet destruction results in thrombocytopenia | Several new antithrombotic agents currently are under investigation. These agents must show an improved efficacy over available agents without an increased toxicity. However, since efficacy and toxicity are so closely related for antithrombotic agents and current drugs are quite effi cacious with only moderate toxicity, the challenge for new agents to reach the clinic is great. Well-designed clinical studies stratified for the risk of recurrent venous and arterial thromboembolism still are needed to determine the optimal intensity and duration of therapy with currently available agents. Since several of the new antiplatelet agents have been used in only a few regimens, clinical trials need to be continued to discover the optimal combinations of drugs to maximize safety and minimize toxicity. In this way, prevention of thrombosis may be improved. The development of portable devices to measure the INR in patients undergoing long-term therapy with oral anticoagulants should permit patient self-management, which may lead to reduction of cost and episodes of hemorrhage.
