Here is a reference to a brilliant review of current aspirin efficacy studies, focusing especially on the prospective random control trial ASPREE published in the September, 2018 New England Journal of Medicine.
Lippi G, Danese E, Favaloro EL. Harms and Benefits of Using Aspirin for Primary Prevention of Cardiovascular Disease: A Narrative Overview. Semin Hemostas Thrombosis 2018. DOI https://doi.org/ 10.1055/s-0038-1675380. ISSN 0094-6176. It appears that aspirin efficacy for primary prevention is going off the table, and the ASPREE trial, contrary to others, shows an association between regular aspirin usage and cancer among people over 65. Sorry, copyright rules prevent me from posting the actual review, please obtain it from your medical library.
Because none of the trials reviewed by Lippi include a laboratory component, I (Geo) have posted a November, 2018 Quick Question asking whether you monitor for aspirin efficacy, and if so, what platform do you use? Please check out our question and provide your answer.