Resistance to low-dose aspirin therapy among patients with acute coronary syndrome in relation to associated risk factors
Journal of Clinical Pharmacy and Therapeutics,  Clinical Article

Salama MM et al. – There is inter–individual variability in response to the antiplatelet effect of standard doses of aspirin (150, 300 mg/day). The response to aspirin 300 mg/day is enhanced in resistant patients when compared to 150 mg/day. There was a significant association between aspirin resistance and atherothrombotic risk factors (diabetes, hyperlipidaemia and obesity).

Methods
  • The antiplatelet effect of 150 mg/day aspirin was studied prospectively in 50 consecutive patients with unstable angina or non–ST–segment elevation myocardial infarction.
  • Platelet aggregation was measured using optical platelet aggregometry and serum thromboxane B2 level.
  • Aspirin resistance was defined as collagen (1 μg/mL) and adenosine diphosphate (ADP) (5 μmol/L)–induced platelet aggregation of ≥40% when compared with control values.
  • Twenty healthy age– and sex–matched individuals were taken as a control group.
  • All patients were subjected to complete medical history (risk factors, medications), thorough clinical examination, ECG, coronary angiography and laboratory investigations including: complete haemogram, coagulation, kidney, liver and lipid profiles, fasting blood glucose and glycated haemoglobin (HbA1C).

Results
  • Eleven of 50 patients (22%) were found to be aspirin resistant.
  • A highly significant difference was found between the mean values of ADP, collagen–induced platelet aggregation percentage and thromboxane B2 level after aspirin 150 mg/day when compared with the corresponding mean values after aspirin 300 mg/day among the resistant patients (66 ± 7•01%, 62 ± 4•34% and 620 ± 64•58 pg/mL, respectively, vs. 26•87 ± 2•85%, 16•5 ± 3•8% and 77 ± 11•3 pg/mL) indicating enhanced response to aspirin after escalating the dose.
  • The presence of atherothrombotic risk factors (hypertension, smoking, family history of ischaemic heart disease and previous MI) were not statistically different between aspirin–resistant and aspirin–sensitive patients.
  • However, there was a highly significant difference between the aspirin sensitive and the resistant patients regarding the other risk factors (diabetes mellitus and dyslipidaemia) (P < 0•01).

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