Inconsistency of different methods for assessing ex vivo platelet function: relevance for the detection of aspirin resistance.

Background: Assays to evaluate platelet function are often interchangeably used to assess "resistance" to aspirin. We compared different platelet function assays in patients treated or untreated with aspirin.
Design and Methods: Platelet function was evaluated in 162 subjects, 85 of whom were not being treated with any antiplatelet drug and 77 of whom were receiving chronic therapy with low-dose aspirin. Platelet Function Analyzer collagen/ADP- and collagen/epinephrine closure times, as well as light transmittance aggregometry in response to ADP, collagen and arachidonic acid (this last in 47 aspirin-treated patients) were determined. In 43 aspirin-treated patients, serum thromboxane B(2) levels were also measured.
Results: In untreated patients, collagen/ADP- and collagen/epinephrine-closure times were correlated with each other (r=0.5, P=0.0001), but did not correlate with ADP- or collagen-induced aggregation. In patients treated with aspirin, collagen/ADP-closure time values were not different from those in untreated patients, while the collagen/epinephrine-closure time was prolonged. ADP-induced aggregation was unaffected by aspirin, while collagen-induced aggregation was reduced. Arachidonic acid-induced aggregation was almost completely suppressed (% maximum light transmittance aggregometry =5 ± 13%). There was, however, no correlation between the various platelet function tests. Serum thromboxane B(2), an index of platelet cyclooxygenase-1 activity, was almost completely suppressed (down to 8 ± 17 ng/mL) in treated patients, and was not correlated with arachidonic acid-, ADP- and collagen-induced aggregation or with collagen/ADP-closure time, but was inversely correlated with collagen/epinephrine-closure time.
Conclusions: There is a high heterogeneity of results of tests evaluating inhibition of platelet function by aspirin, and the results of functional tests do not match biochemical measurement of cyclooxygenase-1 activity. Extreme caution should, therefore, be used in defining "resistance" to aspirin on the basis of the results of these tests.