Resistance to low-dose aspirin therapy among patients with acute coronary syndrome in relation to associated risk factors Journal of Clinical Pharmacy and Therapeutics, 05/04/2012

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).

• 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).

• 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).