Minimal daily dosage of ASA for platelet inhibition.

To the Editor: Acetylsalicylic acid (ASA) has the property of inhibiting cyclo-oxy genase of both platelets and vessel wall intima.' However, a differential inhibition of the cyclo-oxygenase in the platelets has been claimed possible by low doses of ASA. Preservation of the vessel wall pro duction of prostacyclin, which is a potent inhibitor of platelet aggrega tion, would seem a therapeutic benefit. ASA in large doses of one gram daily or more has not shown convincing protective effect against throm boembolism,and the possibility that small doses of aspirin might be more effective remains to be thoroughly investigated. We studied the lowest daily dose of AS A necessary to inhibit platelet aggregation in 40 patients with cerebrovascular disease of a mean age of 61 years. Platelet aggregation was studied in a Born aggregometer after adding sodium arachidonate to platelet-rich plasma. Over a treatment period averaging 118 days, 13 patients showed full inhibition of platelet aggregation on a daily ASA dose of 25 mg. In 85 per cent of the patients the daily requirement was below 75 mg ASA. The highest dose needed was 125 mg daily in 2 patients. It is concluded that the ASA dosage used in the large controlled studies are far in excess of what is needed. For future controlled studies of the effect of ASA on thromboembolism it is suggested to use " minimal daily doses and to monitor the effect on platelet function individually.