A new definition of aspirin non-responsiveness by platelet function analyzer-100 and its predictors

Aspirin non-responsiveness has been described as having a normal closure time (CT) by platelet function analyzer (PFA)-100 assay despite confirmed treatment with aspirin. There is no standard definition of aspirin non-responsiveness by PFA-100, with a variety of cut-off values having been used. We proposed an alternative definition of aspirin non-responsiveness by PFA-100 assay.

One hundred eighty-four patients with diagnosis of stable coronary artery disease or diabetes mellitus were included in the study. Blood samples were drawn before and after the 7 days of aspirin therapy. An individual was labelled as aspirin non-responder if his/her post-aspirin CT was not 2SD above his/her baseline CT, where SD was calculated from the baseline CTs of the study population. Aspirin non-responsiveness was also defined as having a normal post-aspirin CT (< or =193 s) regardless of pre-aspirin CT.

The baseline CT ranged 82-187 s (mean 129.1 +/- 27.5, median 128 s) in the study population. At the end of 1 week of aspirin administration, CT increased to a mean of 260.7 +/- 63.6 s (range 102-301). According to our definition, 28 (15.2%) of 184 patients were aspirin non-responders. Univariate analysis indicated that aspirin non-responsiveness was closely associated with gender (P = 0.012) diabetes (P = 0.006), smoking (P = 0.0496) and hypertension (P = 0.021). Multivariate analysis identified diabetes (P = 0.016) as the only significant independent predictor for the presence of aspirin non-responsiveness. Thirty-four of 184 patients (18.5%) classified as aspirin non-responders according to the second criteria. Seven patients with prolongation of post-aspirin CT more than 2SD were classified as aspirin non-responders by the second criteria. Only 1 patient without prolongation of CT more than 2SD was classified as aspirin responsive by the second criteria.

Definition of aspirin non-responsiveness as post-aspirin CTs < or =193 s might overestimate the prevalence of aspirin non-responsiveness. Nevertheless, definition of aspirin non-responsiveness by PFA-100 must be standardized and its utility as a predictor of cardiovascular events needs to be further investigated.