Our data from 214 patients after cessation of long-term therapy with 100 mg/d ASA demonstrate that the determination of platelet-related primary hemostasis in citrated whole blood with PFA-100 is a reliable and sensitive method for the detection of ASA-induced platelet dysfunction. However, the sensitivity of the method is strongly dependent on concentration of sodium citrate used as anticoagulant. The results of PFA-100 testing show a clearly enhanced sensitivity for ASA when blood samples were collected with 0.129 M rather than 0.106 M sodium citrate. According to sample stability, PFA-100 results can only be confirmed up to 1 hour postcollection when blood was anticoagulated with 0.129 M but not with 0.106 M sodium citrate. Therefore, we recommend that testing with PFA-100 in patients with suspected ASA-induced platelet dysfunction should be performed exclusively in blood collected in buffered 0.129 M sodium citrate.