Twenty-one studies were included for review (17,522 participants): 17 RCTs and four cohort studies. Thirteen RCTs scored between 21 and 25 on the quality checklist, three scored between 16 and 20 and one scored 15. Quality scores for the cohort studies ranged from 11 to 14. Follow-up ranged from one to 11.6 years (median 3.6).
When RCTs and cohort studies were combined, aspirin use in participants with diabetes was not associated with lower all-cause mortality (RR 0.93, 95% CI 0.81 to 1.07; 25 studies) and cardiovascular mortality (RR 0.98, 95% CI 0.76 to 1.25) and lower risk of myocardial infarction (RR 0.84, 95% CI 0.65 to 1.09) and stroke (RR 0.98, 95% CI 0.82 to 1.16) compared to controls. There was evidence of significant statistical heterogeneity for the outcomes of all-cause mortality (Ι²=57%), cardiovascular mortality (Ι²=54%) and myocardial infarction (Ι²=71%).
When stratified by daily dose, there was no significant relationship between aspirin dose and all-cause or cardiovascular mortality. An aspirin dose of more than 325mg daily was associated with a decreased risk of myocardial infarction (RR 0.85, 95% CI 0.73 to 0.99) in participants with diabetes compared to controls. A daily aspirin dose of 100mg or less was associated with a decreased risk of stroke in participants with diabetes (RR 0.81, 95% CI 0.68 to 0.97) compared to controls. There was no evidence of statistical heterogeneity for either analysis (Ι²=0%) but these results were of borderline statistical significance.
Aspirin use was associated with a statistically borderline decreased all-cause mortality in diabetic participants with a prior history of cardiovascular disease (RR 0.82, 95% CI 0.69 to 0.98; 13 studies; Ι²=27%). This appeared to be associated with an aspirin dose of 325mg or less daily (RR 0.77, 95% CI 0.63 to 0.95; Ι²=48%). Aspirin use did not significantly alter all-cause mortality in diabetic participants with no prior history of cardiovascular disease (RR 1.01 95% CI 0.85 to 1.19; seven studies; Ι²=0%).
Where data only from RCTs were included, aspirin use overall was not associated with a decreased risk of stroke in diabetic participants (RR 0.90, 95% CI 0.76 to 1.07; 17 studies). However, aspirin dose of 100mg or less was associated with a significantly lower risk of stroke (RR 0.76, 95% CI 0.63 to 0.92; 10 studies). There was no evidence of significant statistical heterogeneity for these outcomes.
The authors found no evidence of publication bias from visual inspection of the funnel plots.