Effect of l-aspirin on in vitro platelet plug formation in citrated or lepirudin anticoagulated blood.l-aspirin was added in vitro to whole blood (50 mg/L; A) or infused into volunteers (250 mg; B) for determination of collagen-epinephrine (CEPI) closure time (CT) in citrated blood. For the determination of CEPI and collagen-adenosine diphosphate (CADP) CTs in lepirudinised blood, l-aspirin was infused into volunteers (250 mg; C, D). l-aspirin prolonged the CEPI-CT (n = 10) in citrated blood (A, B) whereas in lepirudinised blood the CEPI-CT (n = 8) was less influenced (C) and the CADP-CT (n = 10) was not prolonged in vitro (D). Dotted lines refer to upper normal limits.