We have used the platelet analyzer PFA-100TM to assess the effect of aspirin (ASA) in patients with documented peripheral arterial disease (PAD). Thirty-one previously untreated patients were recruited. Laboratory investigations, including the collagen and adenosine diphosphate closure time (CADP-CT) and the collagen and epinephrine closure time (CEPI-CT) were performed before and 7 days after treatment with 100 mg ASA per day. Five patients were excluded from the final analysis: one patient did not appear for second examination, in one patient type I von Willebrand disease was diagnosed, and three patients with prolonged CEPI-CT admitted the intake of non-steroidal anti-inflammatory drugs. Prior to ASA treatment, CADP-CT was 90 +/- 15 s (range, 67-124 s) and CEPI-CT was 116 +/- 27 s (range, 78-164 s). There was a significant negative correlation between CADP-CT and von Willebrand factor antigen (r = -0.57, P = 0.001). After treatment with 100 mg ASA per day, CADP-CT was not significantly different (96 +/- 22 s; range, 65-158 s). CEPI-CT, however, was prolonged in all patients, compared with pre-ASA values (226 +/- 82 s; range, 89 to > 300 s). In 12 of 26 patients, CEPI-CT was > 300 s and in another four of 26 patients CEPI-CT was prolonged to more than the upper normal range ('responders'). In the remaining 10 patients, CEPI-CT values did not exceed the upper limit of the normal range ('non-responders'). Five non-responders were re-investigated after intake of 300 mg ASA per day for 3 weeks; in none of these was a CEPI-CT > 165 s recorded. We conclude that 40% of PAD patients have an inadequate response to ASA, as determined by the PFA-100TM CEPI-CT. Whether these patients have a reduced benefit from this treatment remains to be investigated.

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